Semiconductor device assembly techniques using preformed planar structures

ABSTRACT

An interposer (preformed planar structure) is disposed between a die and a substrate (which may be another die). Through holes in the interposer facilitate controlled formation of electrical connections between the die and the substrate. In one embodiment, the through-holes in the interposer are filled flush with a resilient plastic conductive material and pressed against raised conductive structures on the die and substrate. The die, interposer, and substrate are maintained in electrical contact under compression. The compressing force can be removed to replace the die. In another embodiment, the interposer has embedded conductive elements which make contact with selected connections between the die and the substrate. Electrical connections between the conductive elements can be selectively effected to provide for &#34;re-wiring&#34; of connections to the die and substrate. Another similar embodiment is directed to using embedded termination networks to provide integral termination for signals between the die and substrate. Various other embodiments are directed to providing grooves on the surface of the interposer to facilitate the flow of a cooling gas, and to forming plastic conductive connections between a die and a substrate in a flip-chip assembly using an interposer.

CROSS-REFERENCE TO RELATED CASES

This is a divisional of application Ser. No. 08/260,078, filed on Jun. 15, 1994, and now U.S. Pat. No. 5,468,681 which is a continuation-in-part of commonly-owned, copending U.S. patent application Nos. 08/105,547 (status pending) and 08/105,269 (status pending), both filed Aug. 12, 1993 by PASCH, et al., and of U.S. patent application Nos. 08/106,157 (now Pat. No. 5,489,804) and 08/105,838 (now Pat. No. 5,347,162), both filed Aug. 12, 1993 by PASCH. All four of these cases are continuations-in-part of commonly-owned, U.S. patent application No. 07/981,096, filed Nov. 24, 1992 (now Pat. No. 5,299,730), which is a continuation of U.S. patent application Ser. No. 07/775,009, filed Oct. 11, 1991 (now Pat. No. 5,168,346, issued Dec. 1, 1992), which is a continuation of U.S. patent application Ser. No. 07/576,182, filed Aug. 30, 1990 (now Pat. No. 5,111,279, issued May 5, 1992), which is a continuation of U.S. patent application Ser. No. 07/400,572, filed Aug. 28, 1989 (status abandoned), all of which were filed by PASCH et al.

TECHNICAL FIELD OF THE INVENTION

The invention relates to semiconductor manufacturing techniques and, more particularly, to the assembly techniques employed in flip-chip and chip-on-board types of manufacture.

BACKGROUND OF THE INVENTION

Dramatic advances have been made in miniaturizing and packaging electronic devices in the time since the first computers were developed. One need only compare the room-sized computers of the early 1950's with today's laptop and notebook computers to appreciate just how dramatic these advances have been. Today's notebook computers are modestly priced, can be less than one inch thick, have a footprint no larger than an 81/2" by 11" piece of paper, operate for hours off of battery power, contain many megabytes of memory, hundreds of megabytes of magnetic storage (hard disk), and execute tens of millions of instructions per second. By way of contrast, the earliest computers cost millions of dollars, could typically fill an entire room, required massive amounts of power and cooling, had very little memory and storage capability, and could execute only about one thousand instructions per second.

This miniaturization and performance increase is due, in large part, to dramatic advances in semiconductor technology, particularly in semiconductor processing and packaging. A single modern semiconductor integrated circuit die no more than one half inch square can contain millions of transistors. Combined with high-density packaging techniques, modern semiconductor device assemblies provide almost unbelievable amounts of function in an almost unbelievably small package. Notwithstanding these tremendous advances in semiconductor processing and assembly techniques, there is still a great demand for even further improvement in package density.

Modern semiconductor device assemblies are typified by a semiconductor die mounted within a die-receiving cavity of a protective package (or on a leadframe, or other substrate). I/O (Input/Output) pads on the die are electrically connected, within the body of the package, to conductive traces or leads which extend to the exterior of the package, where they terminate in conductive leads, pins, pads, balls or fingers to which further electrical connections are made. Typically the package is mounted to a circuit board having a number of other electronic devices mounted thereto, some of which may be other semiconductor device assemblies. The packages serve to dissipate heat from the die, to protect it from moisture, and to provide a durable exterior which protects the die from physical damage.

As semiconductor device geometries (e.g., line width) become smaller, it becomes possible to provide ever greater numbers of transistors on a semiconductor die. Several limitations come about as a result of conventional packaging techniques, however. For example, very large integrated circuits (i.e., containing large numbers of transistors or other integrated electronic components) tend to require large numbers of I/O connection points (many hundreds in some cases). Although these connection points can be placed very densely on a semiconductor die, there are physical limits to how closely they can be spaced on the exterior of a package body. It is not uncommon, due to a large number of connections, that a one half inch square die must be mounted in a 2 to 3 inch square package body in order to provide sufficient space on the package body for the relatively large and coarsely spaced I/O pins.

Further, a large package body can negate many of the speed benefits which result from small device geometries. Large packages adversely affect how closely semiconductor devices can be spaced on a circuit board or substrate. This, in turn, tends to lengthen the conductive paths (wiring) between the semiconductor devices (i.e., the wiring distance between interconnected I/O pads of two integrated circuit dies). In modern high-speed electronic assemblies, wiring delays can have a significant effect on overall performance. As a result, it is often necessary to find ways to package semiconductor devices so that interconnection length (wiring length) is minimized. (Other factors, such as market-driven demands for ever-smaller consumer products, are not insignificant considerations in applying pressure on semiconductor producers to find ways to package semiconductor devices densely.)

"Chip-on-board" and "flip-chip" manufacturing techniques are high-density electronic device assembly techniques which have been developed in response to pressure for high-density electronic packaging techniques. In both of these techniques, one or more unpackaged (essentially bare) semiconductor dies are mounted directly to tiny "printed circuit boards" and wire to one another in close proximity. In chip-on-board assemblies, a semiconductor die is usually "glued" by its back surface to a circuit-board substrate. Electrical connections are then formed to the die with tiny bond wires, similar to the connections formed to a die within the die-receiving area of a conventional package. Unlike the conventional package, however, there is no intermediate package body between the die and the substrate (circuit board). This permits the die to be place very closely to other electronic devices (including other dies) and for interconnection length to be minimized. After mounting and connection, a protective layer (e.g., an epoxy glob or other suitable protective covering) is disposed over the die and its exposed connections (e.g., the bond wires).

"Flip-chip" manufacturing techniques are similar in some respects (efficient use of area) to chip-on-board techniques, but in flip-chip assemblies, the substrate itself can be another die. Flip-chip techniques involve soldering one or more semiconductor (silicon) chips (one is discussed), in face-to-face relationship, to a substrate (or to another die). Typically, solder balls (otherwise known as pads or bumps) are formed (raised above the planar surface of the chip and substrate) on facing surfaces of both the chip and the substrate at intended points of contact between the two, liquid flux (rosin) is often applied to the face of the chip and/or substrate, the chip is mechanically held in register with the substrate, and the chip and the substrate are subjected to elevated temperature to effect soldering, or fusion of the solder balls on the chip and the corresponding solder balls on the substrate.

The "solder balls" on either the chip or substrate, typically those on the substrate, may be solderable metallized surfaces. The soldering process may be carried out in a reducing atmosphere. A typical flip-chip structure is shown in FIG. 1, and is discussed in greater detail hereinafter.

Previous systems of rigid attachment of chips to chucks have been used for chip alignment, but they must allow some degree of compliance because the chips tend to change relative alignment during soldering by surface tension between the solder balls. The addition of liquid flux to the chip/substrate (flip-chip) assembly creates capillary attraction between the chip and the substrate, which serves to mis-align the chip with respect to the substrate. This is illustrated in FIG. 2, and is discussed in greater detail hereinafter. Further, much of the flux that is applied to the flip-chip assembly is wasted. Still further, the dimension of the remaining gap between the chip and the substrate and the mechanical properties of the solder joints formed by the solder balls and corresponding solder balls tends to be indeterminate.

As discussed hereinabove, direct assembly of semiconductor dies into flip-chip type assemblies can be difficult. When successful, however, such assemblies can provide extremely dense electronic assemblies. In some cases, this high density can create another problem: heat build-up. Whereas in "traditional" packaging techniques the package body and spacing between packages provides for a certain amount of natural, inherent heat dissipation, flip-chip assemblies place chips in very close proximity. This can cause localized "hot-spots" within the flip-chip assembly, particularly where very high switching speeds are involved.

Other problems derive from the close spacing of dies in flip-chip assemblies. With circuits and systems built from "traditionally" packaged electronic devices, there is usually sufficient board space available that providing termination networks for high-speed signals and busses does not require significant additional space (as a portion of the total space required for the circuit or system). With flip-chip assemblies, however, since the total amount of space is so small, any additional space required for termination networks (or other discrete active or passive electronic circuitry) can represent a significant increase in total space.

Accordingly, the present invention is directed to solving these and other related problems (e.g.: alignment during assembly of flip-chip devices; application, control, and removal of solder flux; heat dissipation within dense semiconductor assemblies; space required by termination networks; etc.). Since many of these problems apply to a relatively broad class of dense semiconductor assemblies, the present invention is more broadly (i.e., than chip-to-chip) directed to connecting one or more semiconductor "chips" (dies) to one or more "substrates" such as other semiconductor dies, printed circuit (or wiring) boards, and the like. The resulting assembly is termed a "flip-chip structure" for the purposes of the present invention.

DISCLOSURE OF THE INVENTION

It is therefore an object of the invention to provide a flip-chip manufacturing technique which reduces capillary action, and hence misalignment, between a chip and a substrate.

It is a further object of the invention to provide a flip-chip manufacturing technique which requires the use of less flux, and which controls the position of the flux between the chip and the substrate.

It is a further object of the invention to provide a flip-chip manufacturing technique which provides a controlled spacing between chips and the substrate.

It is a further object of the invention to provide a flip-chip manufacturing technique which provides solder joints having predictable and tailorable mechanical characteristics.

It is a further object of the invention to provide a flip-chip manufacturing technique that simplifies the face-to-face joining of the chip and substrate.

It is a further object of the invention to provide a flip-chip manufacturing technique which yields repairable flip-chip assemblies wherein a defective die can be replaced.

It is a further object of the invention to provide a technique for flip-chip assembly which provides for "programmability" of the interconnections between a die and a substrate.

It is a further object of the invention to provide a technique for flip-chip assembly which facilitates active gas cooling of the assembly.

According to the invention, a preformed planar structure is interposed between the chip(s) and the substrate in a flip-chip structure. The preformed planar structure establishes a minimum gap between the chip(s) and the substrate.

According to a feature of the invention, liquid flux is applied to the preformed planar structure in order that flux is selectively applied to the solder balls (pads) on the chip and the substrate.

In an embodiment of the invention, the preformed planar structure is provided with through holes in registration with the solder balls (pads) on the chip(s) and the substrate. In this embodiment, liquid flux selectively fills the through holes for delivery to the solder balls during soldering. The through holes also aid in maintaining registration of the chip(s) and the substrate.

According to an aspect of the invention, the through holes are sized to establish a predetermined mechanical structure of solder joints formed by the solder balls when fused together.

According to an aspect of the invention, the preformed planar structure has a planar core and opposing planar faces. The core is formed of thermosetting organic resin, such as polyimide, or non-organic material such as alumina, polished sapphire, beryllium oxide, aluminum or aluminum nitride. The planar faces of the preformed planar structure are formed of thermoplastic resin or thermosetting material, such as polyacetal, epoxy (epoxy resins) or polystyrene. The preformed planar structure tends to draw the chip(s) together to the substrate, establishing a flip-chip structure or mechanical integrity.

According to an aspect of the invention, the preformed planar structure has a thickness of 5-50 microns, preferably on the order of 20-30 microns.

Additional and further embodiments and variations of a preformed planar structure are set forth below, with respect to the descriptions of FIGS. 6-18.

Other objects, features and advantages of the invention will become apparent in light of the following description thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a typical, prior art flip-chip structure.

FIG. 2 is a cross-sectional view of a prior art flip-chip assembly illustrating capillary action caused by liquid flux, resulting in the misalignment of a chip with respect to a substrate.

FIG. 3 is an exploded cross-sectional view of a flip-chip assembly, prior to soldering, according to a technique of the invention.

FIG. 4 is a perspective view of a plastic standoff element (preformed planar structure) employed in the technique of FIG. 3.

FIG. 5 is a perspective view of an alternate embodiment of a standoff element suitable to be employed in the technique of FIG. 3.

In the description of further embodiments of a preformed planar structure that follows, the preformed planar structure may alternately be referred to as an "interposer".

FIG. 6 is an exploded cross-sectional view of a flip-chip assembly incorporating a preformed planar structure (interposer), according to the invention.

FIG. 6a is a cross-sectional view of another embodiment of a preformed planar structure, similar to that shown in FIG. 6, according to the invention.

FIG. 6b is a cross-sectional view of another embodiment of a preformed planar structure, similar to that shown in FIG. 6a, according to the invention.

FIG. 6c is a cross-sectional view of an encapsulated semiconductor die assembled to a preformed planar structure, according to the invention.

FIG. 7 is a cutaway view of a preformed planar structure including a ring-shaped array of angled through holes, according to the invention.

FIG. 7a is a top view of the preformed planar structure of FIG. 7.

FIG. 7b is a top view of a preformed planar structure with a rectangular array of angled through holes, according to the invention.

FIG. 8 is a cross-sectional view of a semiconductor device assembly employing a preformed planar structure as a connection "pitch adapter", according to the invention.

FIG. 8a is a cross-sectional view of a semiconductor device assembly employing a multi-layer preformed planar structure as a connection pitch adapter, according to the invention.

FIG. 9a is a view of a semiconductor device assembly employing dissolvable preformed planar structures, according to the invention.

FIG. 9b is a view of a the semiconductor device assembly of FIG. 9a, after dissolving the preformed planar structures.

FIG. 10a is a view of a ring-shaped preformed planar structure, according to the invention.

FIG. 10b is a view of a gapped ring-shaped preformed planar structure, according to the invention.

FIGS. 10c and 10d are top and side views, respectively, of a portion of a leg of a kerfed ring-shaped preformed planar structure (interposer), according to the invention.

FIG. 10e is a close-up top view of a portion of a leg of another embodiment of a kerfed ring-shaped preformed planar structure (interposer), according to the invention.

FIG. 10f is a side view of the kerfed interposer of FIG. 10e.

FIG. 11a is a cross-sectional view of a semiconductor device assembly employing a preformed planar structure (interposer) with an embedded conductive trace, probe finger, and embedded electronic components, according to the invention.

FIG. 11b is a cross-sectional view of a semiconductor device assembly similar to that of FIG. 11a, but employing selectively formed conductive bump contacts to determine which of a set of possible connections are made..

FIG. 11c is a schematic diagram showing how programmability of electrical connections in a preformed planar structure is accomplished.

FIG. 11d is a side view of a semiconductor device assembly employing a preformed planar structure (interposer) wherein embedded conductive traces exit the interposer and form conductive leads of the assembly, according to the invention.

FIG. 11e is a top view of a preformed planar structure (interposer) similar to that of FIG. 11c, wherein conductive leads embedded in the interposer are connected to an external "ribbon-cable", according to the invention.

FIG. 12a is a cross-sectional view of a semiconductor device assembly employing noble metal conductors embedded in a preformed planar structure to prevent electro-galvanic corrosion of dissimilar solder bump contacts and to form other electrical connections within the preformed planar structure, according to the invention.

FIG. 12b is a cross-sectional view of a semiconductor device assembly combining the techniques of FIGS. 11b and 12a to provide electro-galvanic corrosion protection combined with integral conductors and/or embedded electronic elements, according to the invention.

FIG. 12c is a plan view of an interposer, such as that of FIG. 12b, looking into one of the through holes.

FIG. 13a is a cutaway side view of one embodiment of a stepped preformed planar structure, according to the invention.

FIG. 13b is a cutaway side view of another embodiment of a stepped preformed planar structure, according to the invention.

FIGS. 14a-d are side views of various embodiments of transparent, colored, clear, and translucent preformed planar structures (interposers) according to the invention.

FIG. 15a is a side view of a flip-chip semiconductor device assembly, according to the invention.

FIG. 15b is a side view of a multi-tier flip-chip semiconductor device assembly, according to the invention.

FIG. 15c is a cutaway side view of a flip-chip semiconductor device assembly, according to the invention.

FIG. 16 is a cross-sectional view of a repairable semiconductor device assembly utilizing a die and substrate with raised conductive structures a preformed planar structure with resilient plastic conductors, according to the invention.

FIG. 17a is side view of a semiconductor device assembly utilizing a preformed planar structure with integral cooling channels, according to the invention.

FIG. 17b is a perspective view of a semiconductor device assembly utilizing an extended preformed planar structure with integral cooling channels, according to the invention.

FIGS. 18a-18e are views of a preformed planar structure at various steps of a method for forming conductive plastic connections therethrough, according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 illustrate a prior art technique of assembling flip-chips 10. The completed flip-chip structure 10 includes one or more silicon chips 12 (two of such chips are illustrated) mounted in face-to-face relationship to a larger silicon chip or substrate 14 in the following manner. Solder balls (or pads) 16 are formed on the face 12A of the chip 12, and solder balls (or pads) 18 are formed on the face 14A of the substrate in corresponding positions (i.e., to align with the solder balls 16 of the chips).

It should be understood that the solder balls on either the chip 12 or on the substrate, typically, the corresponding solder balls 18 on the substrate, may simply be solderable metallized surfaces.

It should also be understood that balls (of dollops) of conductive epoxy or polymer could be used in lieu of solder balls. In the main hereinbelow, solder balls are discussed.

Liquid flux 20 (shown in FIG. 2 only) is applied to the face 14A of the substrate, prior to bringing the chips 12 in face-to-face relationship with the substrate 14. The chips 12 are placed in face-to-face relationship by mechanical means, such as with a chuck (not illustrated), and the temperature of the chips and substrate is elevated sufficiently to cause the solder balls 16 on the face 12A of the chips 12 to "fuse", forming solder joints, with the corresponding solder balls 18 on the face 14A of the substrate 14.

As illustrated in FIG. 2, the liquid flux 20 fills a gap 22 between the faces 12A of the chips 12 and the face 14A of the substrate 14, and also fills an area on the face 14A of the substrate 14 between the chips 12. Capillary action and/or surface tension created by the presence of the liquid flux 20 causes the chips 12 to migrate to the center 24 of the substrate 14 during the soldering process, resulting in a lack of registration between the chips 12 and the substrate 14, and hence between the solder balls 16 and the corresponding solder balls 18, respectively. This tendency of the chips to become misaligned during soldering is augmented by any initial lack of planarity between the chips 12 and the substrate 14, and is extremely disadvantageous in the assembly of flip-chips. For instance, it becomes extremely difficult to characterize the mechanical configuration of the solder joints not only because of the difficulty in maintaining accurate registration, but also because of the somewhat unpredictable dimension of the gap 22 between the chips and the substrate. This can adversely affect flip-chip throughput in the manufacturing process. Further, as is evident from FIG. 2, much more flux is used than is required to effect clean solder joints between the solder balls 16 and corresponding solder balls 18, respectively.

FIG. 3 illustrates a technique of assembling flip-chips 30. In a manner similar to that previously discussed, the flip-chip assembly 30 includes one or more silicon chips 32 (two of such chips are illustrated) ultimately mounted in face-to-face relationship to a larger silicon chip or substrate 14 in the following manner. Solder balls 36 are formed on the face 32A of the chip 32, and solder balls 38 are formed on the face 34A of the substrate 34 in corresponding positions. Notably, liquid flux is not applied to the face 34A of the substrate 34, prior to soldering the chips 32 thereto. Nor is liquid flux required to be applied to the faces 32A of the chips 32.

Prior to soldering the chips 32 to the substrate 34, a preformed planar structure 40 (otherwise termed a "stamp" or "plastic standoff element", and discussed in greater detail hereinafter), of similar planar dimension as the chip 32, is interposed between the chips 32 and the substrate 34. The planar structure 40 is provided with through holes 42 in positions corresponding to the positions of the solder balls 26 and 38, respectively. Inasmuch as the solder balls 36 are typically located just within the perimeter of the chips 32, the through holes 42 would be located just within the perimeter of the planar structure 40.

Prior to soldering, the planar structure 40 is dipped (not illustrated) into a solution (bath) of liquid flux, such as rosin material, and is allowed to dry, as shown at 44. In this manner, the planar structure 40 receives selectively deposited rosin preferentially within the holes 42, in registration with the corresponding solder balls 32 and 34, and there will be very little, if any, flux on the planar surface of the preformed planar structure. The capillary action of liquid solution in small holes draws the bulk of the liquid flux material into the through holes 42, which are disposed in register with the solder balls, to effect successful solder bonding.

This selective application of flux to the solder balls has numerous advantages: there will be very little flux on the surfaces of the planar structure; a minimum amount of rosin is used to flux the solder balls; extra (waste) flux is kept out of the flip-chip assembly; and relative motion of the chips otherwise caused by the capillary action and/or surface tension of such excess molten flux will be minimized.

By successive dilution of the flux bath into which the planar structure is dipped, an optimal amount of flux can be empirically determined for particular applications that will both successfully flux the solder balls and minimize flux usage, and the aforementioned problems inherent therein.

Thus, the chips 32 are more easily and accurately held in place by mechanical means, such as with a chuck (not illustrated) during soldering, resulting in increased throughput (yield) of flip-chips in the manufacturing process. Further, the holes 42 in the planar structure 40 assist in maintaining registration of the solder balls 36 and corresponding solder balls 38, respectively, and hence alignment of the chips 32 with respect to the substrate 34.

FIG. 4 shows, in further detail, the preformed planar structure 40 used in the technique of FIG. 3. References to FIG. 3 are made in the following paragraphs.

The preformed planar structure 40 includes a planar core 46 formed of a material such as thermosetting organic resin or non-organic material (e.g. aluminum sheet, alumina sheet, beryllium oxide sheet). Laminated to both opposing faces of the core 46 are planar layers (faces) 48 formed of thermoplastic (resin) or thermosetting "skin" which can be expected to soften significantly at the elevated temperatures employed for solder reflow in the flip-chip bonding process. This softening and consequent shrinking of the thermoplastic resin skin (and hence shrinkage of the overall planar structure) will allow and encourage the chips to draw or grow closer to the substrate in response to surface tension caused by the molten solder balls and surface tension of the skin itself. This "growing together" of the chips and substrate, in other words diminution of the gap therebetween, is desirable to ensure that each and every solder ball has an opportunity to grow together and successfully fuse. Again, this enhances flip-chip throughput (yield) in the manufacturing process. It is also important to regulate the amount of growing together in that the requirements for solder ball shape may require a planar structure which does not generate a structure of minimum surface area.

The thermoplastic faces 48 will re-solidify after soldering (upon reduction in temperature) and create a cushion for the faces 32A and 34A of the chips 32 and substrate 34, respectively. Simultaneously, the shrinkage of the planar structure 40, especially the thermoplastic faces 48 thereof, will have the effect of drawing the chips together as they cool off to room temperature after soldering. In this manner, the solder balls are mechanically kept in contact with the chips and substrate, respectively, as well as with each other.

The core 46 of the preformed planar structure exhibits good thermal conductivity, and is formed of a rigid thermosetting organic resin or non-organic material, such as polyimide, polished alumina, polished sapphire, beryllium oxide, aluminum/or aluminum nitride. A suitable polyimide is available from CIBA-GEIGY Corporation, Santa Clara, Calif., in their Probimide (TM) 300 or 400 Series, or Selectilux (TM) HTR 3, microelectronic materials. The faces 48 of the preformed planar structure 40 are formed of a thermoplastic material such as polyacetal, epoxides or polystyrene. It is advantageous that the preformed planar structure exhibit hermeticity and that it does not wick the liquid flux. The overall thickness of the preformed planar structure 40 is on the order of 5-50 microns, preferably 30-30 microns, and the preformed planar structure acts as a physical barrier standoff between the chips and the substrate.

A synergistic effect results from the use of the preformed planar structure which effectively eliminates flux from the faces of the chips and substrate by selectively causing the flux to be deposited on the solder balls and corresponding solder balls, respectively. Inasmuch as the faces of the chips and substrates are relatively clean, any adhesion of the planar structure (notably the "skin") thereto effects a mechanical connection of the solder balls and corresponding solder balls irrespective of soldering. (Albeit, the adhesion and shrinkage become effective at the elevated temperature experienced during soldering). This satisfies the adage that, "good mechanical joints lead to good solder joints".

The preformed planar structure 40 serves as a plastic standoff element to determine the size of the gap between the chips and the substrate. Evidently, the relatively solid core 46 of the planar structure 40 sets a relatively rigid lower limit on the amount that the chips can grow (draw) together to the substrate as the solder balls and corresponding solder balls melt and fuse together.

As mentioned hereinbefore, in prior art flip-chip manufacturing techniques the mechanical properties of the solder joints remain somewhat indeterminate. By use of the planar structure 40 in the manufacturing process, flip-chip structures can be formed without the usual concerns about solder ball bond rigidity. Evidently, the through holes 42 form a generally cylindrical "mold" of predetermined dimension wherein the solder joints are formed. One may view the resulting solder joint formed therein by the solder balls and corresponding solder balls as a mechanical structure (not illustrated) or predetermined dimension, and calculate the resulting mechanical properties thereof, such as rigidity (or elasticity), shear strength, tensile strength, bending moment, etc.

FIG. 5 is a perspective view of an alternate embodiment of a preformed planar structure, or standoff element 50, suitable to be employed in the technique of FIG. 3. References to FIG. 3 are made in the following paragraphs.

With respect to the materials used to form the core 56 and faces 58, and the thickness thereof, the standoff element 50 is similar to the standoff element 40 shown in FIG. 4. However, rather than having through holes 42 in alignment with the solder balls 32 and corresponding solder balls 34, the standoff element 50 may be provided with corner cutouts 52 in alignment with the solder balls 32 and corresponding solder balls 34, and is sufficiently sized so that the solder joints are formed just outside its perimeter. The surfaces of the corner cutouts 52 can be left relatively rough (as compared with the faces 58) in order that liquid flux 54 tends to adhere thereto (as opposed to draining off the faces 58). As discussed with respect to the planar structure 40, the liquid flux is applied to the planar structure 50 by dipping the planar structure in a bath of liquid flux which is allowed to dry thereon. Under the elevated temperatures employed for solder bonding, the flux will be delivered to the solder balls 32 and to the corresponding solder balls 34.

The advantages of the standoff element (preformed planar structure) 50 are similar to those of the standoff element 40 with respect to forming a gap of predetermined dimension between the chip and the substrate, aiding in drawing together the chip and the substrate, mechanically drawing together the solder balls and the corresponding solder balls, requiring less flux to effect soldering and, to a lesser extent aiding in maintaining alignment of the chip and the substrate and alleviating the usual concerns about solder ball bond rigidity.

The preformed planar (layered) structure cushions and draws and holds (upon re-solidifying) the chip and the substrate together in the flip-chip manufacturing process. This improves the mechanical integrity of the flip-chip assembly and increased the resistance thereof to loss of electrical contact between the solder balls. This is important in that the solder balls themselves serve as the mechanical point of attachment between the chip and the substrate. Chip (to substrate) draw together is controlled, and a permanent tension is created between the chip and substrate.

The invention solves the problem of using too much flux and having the position of the chips change during bonding (soldering). The invention allows the use of an absolute minimum of flux so subsequent cleaning of the flip-chip assembly is simplified.

SUMMARY OF PARENT CASES

The aforementioned commonly-owned parent U.S. patent application Ser. Nos. 07/981,096, 07/775,009 (U.S. Pat. No. 5,168,346), 07/576,182 (U.S. Pat. No. 5,111,279) and 07/400,572 describe a preformed planar structure which is suitable to be interposed between a semiconductor die and a substrate. In one embodiment, the preformed planar structure is provided with through holes, that are ultimately filled with a conductive material. Furthermore, the preformed planar structure may be formed as a multi-layer structure having a planar core and opposing planar faces. The core may be formed of thermosetting organic resin, such as polyimide, or of non-organic material such as alumina, polished sapphire, beryllium oxide, aluminum nitride or aluminum. The planar faces of the preformed planar structure may be formed of thermoplastic resin or thermosetting material, such as polyacetal, epoxide resin or polystyrene. Methods of mounting a chip (e.g., semiconductor die) to a substrate (e.g., another semiconductor die), using the preformed planar structure are discussed.

Generally, as described in parent, commonly-owned U.S. Pat. Nos. 5,111,279 and 5,168,346, the preformed planar structures serve at least two purposes: (1) the through holes or cutouts allow for selective fluxing and ensure predictable mechanical characteristics of joints formed by solder balls, and (2) the preformed planar structure provides a prescribed standoff (separation) between the chip and the substrate, and also helps draw them together in intimate contact with the preformed planar structure.

In the discussion that follows, the preformed planar structure is often referred to as an "interposer", especially in those cases where it is literally interposed between a chip and a substrate. Further, in the discussion that follows, it should be understood that the substrate could be a printed circuit boards, another chip or chips, or the like. It should further be understood that the chip could be a complete packaged semiconductor device.

Use of the preformed planar structure as a pitch-spreader

The preformed planar structures of commonly-owned parent U.S. Pat. Nos. 5,111,279 and 5,168,346 generally disclose an interposer (preformed planar structure) for forming solder joints between a chip and a substrate having the same pitch, or spacing, of solder bumps. Generally, the through holes or cut outs extended perpendicularly through the preformed planar structure.

It is an object of the present invention to provide a preformed planar structure suitable for interposing between, and aiding in joining together, a chip having an array (pattern) of solder bumps at one pitch (spacing from one another) and a substrate having an array of solder bumps disposed at another, dissimilar pitch.

It is an object of the invention to provide an improved technique for packaging semiconductor dies, and for interconnecting the packaged dies to external systems.

According to the invention, one face of a preformed planar structure (interposer) is provided with an array of holes disposed at one pitch (spacing) for aligning with the solder bumps of a chip, and the other face of the preformed planar structure is provided with an array of holes at another pitch for aligning with solder bumps on a substrate. Angled or bent through holes through the preformed planar structure extend from the one holes to the other holes, and may be partially or fully filled with a conductive material prior to assembling the chip to the substrate.

According to the invention, a preformed planar structure is interposed between at least one die and an underlying substrate. (The underlying substrate being part of an external system, such as a printed circuit board). The preformed planar structure is provided with at least one through hole. At least one electrical connection is made from the at least one die, through the at least one through hole to connection points on the underlying substrate.

In one embodiment of the invention, the die is flipped face-down onto a face of the preformed planar structure, and solder balls on the face of the die align with the through holes on the face of the preformed planar structure. Circuit elements may be formed on the face (obverse) or back (reverse) side of the die. Typically, the circuit elements would be formed on the face of the die.

According to an aspect of the invention, the (at least one) through holes in the preformed planar structure are pre-filled, or partially pre-filled, with conductive material, such as metal, solder, or conductive polymer or epoxy, such as silver-filled epoxy.

According to a feature of the invention, the reverse (opposite the die) side of the preformed planar structure is provided with ball bumps, or the like, for making connections from external systems (via the underlying substrate) to the die. In this case, the preformed planar structure forms a significant part of the package body surrounding and/or supporting the die.

According to a feature of the invention, the through holes are partially pre-filled with conductive material, and are then pre-fluxed for subsequent attachment of the die and formation of the ball bumps.

According to an aspect of the invention, the through holes are disposed at various angles through the preformed planar structure so that they have a first, relatively close spacing on the face of the preformed planar structure receiving the die (for making connections to an array of closely-spaced solder bumps on the face of the die), and have a second, relatively far-apart spacing on the face of the preformed planar structure making connection to external systems. The connection to external systems would typically be achieved by means of a printed circuit board (PCB) which would have connection points disposed at a greater pitch than the solder bumps on the die.

According to another embodiment of the invention, at least one die is mounted to a preformed planar structure having at least one through hole. At least one connection to the die is made through the at least one through hole. Additional connections to the die are made on the side of the die facing away from the preformed planar structure, such as by wire bonding.

According to an aspect of either embodiment of the invention, the preformed planar structure has a planar core and opposing planar faces. The core is formed of thermosetting organic resin, such as polyimide, or non-organic material such as alumina, polished sapphire, beryllium oxide, aluminum or aluminum nitride. The planar faces of the preformed planar structure are formed of thermoplastic resin or thermosetting material, such as polyacetal, epoxy (epoxide resins) or polystyrene.

Other objects, features and advantages of the invention will become apparent in light of the following description thereof.

FIG. 6 shows (exploded) a die 602 flipped onto a face 604 of a preformed planar structure 606. The preformed planar structure 606 is similar to the previously described structures (see, e.g., the structure described as 40, FIG. 3), but the through holes 608 (compare with through holes 42, FIG. 3) are at least partially filled with a conductive material 610. For example, the conductive material 610 is recessed below the die-side 604 of the preformed planar structure. In this manner, conductive balls (raised conductive bumps) 612 disposed on the face 614 of the die will partially enter the recesses 616 formed in the top surface 604 of the preformed planar structure 606. Additionally, because the conductive material is recessed, flux can be selectively applied to the preformed planar structure, depositing itself exclusively in the recesses, in a manner similar to that set forth with respect to the preformed planar structure 40 (FIG. 3).

The conductive material 610 may be metal, which will fuse with the balls 612, which may be formed of solder. In this manner, a solder joint of predetermined mechanical structure is formed in the recess, similar to what has been described above with respect to FIGS. 3-5.

Alternatively, the conductive material may be a conductive polymer, and the balls 612 may also be formed of a conductive polymer, in which case there will be an adhesion between the balls 612 and the conductive material 610.

It is within the scope of this invention that the conductive material 610 is not recessed below the surface 604 of the preformed planar structure, but is flush with same, as shown in FIG. 6a, or extends slightly above the surface of same, as shown in FIG. 6b.

In a similar manner, the conductive material 610 may extend only partially to the opposite surface 618 of the preformed planar structure 606, as shown in FIG. 6, and the resulting recesses can be pre-fluxed in a manner similar to that described with respect to the preformed planar structure 40 (FIG. 3). The recesses formed by the partially-filled through holes will allow self-registration of conductive balls (raised conductive bumps) 620 on the surface of an underlying substrate 622 to self-self align with the preformed planar structure (with die mounted thereto). The underlying substrate is, for example, a printed circuit board with wire traces (not shown), and electrically connects the die to external components or systems (not shown).

As shown in FIG. 6, solder balls 621 may be formed on the bottom surface of the preformed planar structure.

As shown in FIG. 6c, the preformed planar structure 606 is preferably larger than the die 602, and the die, once joined to the preformed planar structure, can be encapsulated with epoxy 624, or the like. In this manner, the preformed planar structure 606 forms part of a package body enclosing the die. The epoxy 624 forms the remainder of the package body. The preformed planar structure of FIG. 6c would preferably be provided with solder balls such as those shown in FIG. 6 at 621.

The preformed planar structure 606 is preferably thicker than the previously described preformed planar structures (e.g., 40, 50, FIG. 3), and preferably will provide adequate structural support for the die, especially when used in the manner set forth with respect to FIG. 6c. Additionally, a thicker (e.g., than that of FIG. 3) preformed planar structure may be used as a "pitch adapter" for connecting conductive bumps (612) of the die, at a relatively fine pitch, to conductive bumps (620) of an underlying substrate at a relatively coarse pitch.

FIG. 7 shows a cutaway portion of a relatively thick preformed planar structure (interposer) 700 having one surface 702 upon which a die (not shown) will be mounted, and an opposite surface 704 which will be mounted to an underlying substrate (not shown). (The remaining outline of the preformed planar structure 700 is shown in dashed lines 700a.) The preformed planar structure 700 is similar to that described with respect to FIGS. 6, 6a-6c, with the exception that the through holes 710 are arranged at a range of angles with respect to the surfaces 702, 704, such that at least some of the openings (e.g., 716) of the through holes 710 in the bottom surface 704 are not directly under (i.e., are offset from) corresponding openings (e.g., 706) in the top surface 702 of the interposer 700. In the preformed planar structures described hereinabove (40, 606), the through holes were generally all disposed at ninety degrees ("normal") to the surfaces of the preformed planar structure, such that the bottom openings of the through holes were located generally directly beneath the corresponding top openings.

By angling the through holes 704, at a range of angles, the through holes can be made to exit the one surface 702 of the preformed planar structure 700 at one spacing ("pitch"), for example in a linear array matching a pattern of closely spaced conductive bumps arranged around the periphery of a die, and can be made to exit the other surface 704 of the preformed planar structure 700 at a different, larger pitch, for matching a pattern of conductive bumps (or pads) on an underlying substrate (e.g., PWB), which may be arranged in a rectangular array of rows and columns.

FIG. 7a shows a top view of the preformed planar structure 700, and is illustrative of relatively close spacing of the top openings 706 of the through holes 710 as they exit the die-mounting face 702 of the preformed planar structure. Dashed lines indicate the angled trajectory of the through holes 710 through the preformed planar structure 700, ending in bottom openings 716 which are relatively more widely spaced than the top openings 706.

FIG. 7b shows a top view of a similar preformed planar structure 700'. Whereas the preformed planar structure 700 of FIG. 7 showed through holes 710 the top opening of which were arranged in rows, the preformed planar structure 700' has through holes 710' with top openings 706' arranged in a rectangular array on a top (die mounting) surface 702 thereof. As indicated by dashed lines, the through holes 710' generally "fan outward" as they pass through the preformed planar structure 700', ending in bottom (substrate-side) openings 716' which are generally wider spaced than the top side openings 706'.

The through holes 710 or 710' are filled with conductive material (not shown) in one of three ways at each surface: (1) the conductive material is recessed below the surface (compare FIG. 6); (2) the conductive material is flush with the surface (compare FIG. 6a); (3) the conductive material extends out of the surface (compare FIG. 6b). Ball bumps may suitably be applied to the non-die side of the pitch-spreading interposer, and a die mounted thereto can be encapsulated as in FIG. 6c.

The preformed planar structure of this invention can be advantageously applied to situations where it is desirable to package semiconductor dies and attach the packaged dies to underlying substrates having a rectangular array (rows and columns) or ring-shaped array (e.g., as in FIG. 7a) of connection points.

The preformed planar structure (interposer) of this invention can also be advantageously applied to situation where it is necessary to accommodate less regular arrangements of connection points. For example, a substrate might have connection points disposed in a zig-zag (or seemingly random) pattern to mate with corresponding solder bump contacts on a semiconductor die arranged in a linear configuration. By providing a preformed planar structure similar to that described with respect to FIG. 7a, but having bottom side (substrate side) through hole openings matching the zig-zag pattern and top side (die-side) through hole openings in a linear arrangement, the die can be readily adapted, i.e., connected, to the substrate. In other words, the interposer of this invention can serve not only as a pitch spreader, but can also be advantageously employed as an adapter to join connection points (e.g., of a die and a substrate) having completely dissimilar patterns. It will be readily appreciated by one of ordinary skill in the art that many different types of connection point adaptations can be accommodated by the present invention.

It is possible to mount a die to one face of a printed circuit board, for example using wire bonding techniques, and to provide the other face of the printed circuit board with solder bumps (e.g., a two-dimensional rectangular array of raised contacts). Generally, the printed circuit board is provided with plated vias holes connecting wiring traces on the one (die) side of the board to the other (ball bump) side of the board. Additionally, there are traces on the bottom side of the board connecting the plated vias to the ball bumps. An inherent problem with the traces and ball bumps on the other (ball bump) side of the board is that the plated vias are generally arranged around the periphery of the board, and must navigate through adjacent ball bumps, especially to connect to ball bumps located in central areas of the array. Consequently, the ball bumps must be arranged with rather wide spacing, to accommodate one or more (typically more) traces passing therebetween.

By using the preformed planar structure (e.g., 700) of the present invention, there is no need for traces on the bottom (away from the die) side of the package body. As mentioned hereinbefore, the bottom side of the preformed planar structure may be provided with conductive ball bumps, for surface-mounting the packaged die to a printed circuit board (compare FIG. 6).

Preferably, the face of the preformed planar structure to which the die is attached is provided with a thermoplastic layer, or the like (compare 40), which will have the effect of drawing the semiconductor die into intimate contact with the preformed planar structure.

FIG. 8 shows an alternate embodiment of the invention, wherein a die 800 is mounted to a face of the preformed planar structure 800, and is connected with bond wires 804 thereto. In other respects, the preformed planar structure may be similar to any of the preformed planar structures (606, 700) set forth hereinabove. For example, the through holes 806 can be formed at angles with respect to the faces of the preformed planar structure.

It will be readily appreciated by one of ordinary skill in the art that any of the above-mentioned preformed planar structures can be fabricated as a multi-layer structure. FIG. 8a shows an example of such a multi-layer structure, similar to that shown in FIG. 8, except that three laminated layers 802a, 802b, and 802c form the preformed planar structure. In this case, through holes can be "stepped" (they can follow non-linear paths). A through hole consisting of three parts, 806a, 806b, and 806c, is shown extending through the three layers 802a, 802b, and 802c. The top-most portion 806a (nearest the die 800) of the through hole extends vertically (as depicted) through the top-most layer 802a of the preformed planar structure. A second portion 806b of the through hole extends horizontally through the second layer 802b. A bottom-most portion 806c of the through hole extends vertically through the bottom-most layer 802c of the preformed planar structure. Techniques similar to those used for multi-layer printed circuit boards can be used to form the individual portions of the through holes in the individual layers, and to fill them with conductive material. The end result is similar to that shown in FIGS. 7a-7c and 8, where the bottom opening (substrate side) of the through hole need not be directly under the top opening (die side).

Dissolvable Preformed Planar Structure

The preformed planar structures (interposers) of commonly-owned parent U.S. Pat. Nos. 5,111,279 and 5,168,346 generally disclose a "spacer" or "interposer" between a semiconductor die and another die or substrate. Evidently, when a chip (or chips) is assembled to a substrate, as indicated by FIG. 3, the preformed planar structure (40, 50) remains interposed between the chip(s) and the substrate, and may provide for entrapment of contaminants. Contaminants in a flip-chip structure (assembly) are generally undesirable, and may lead to device failure.

It is therefore an object of this embodiment of the invention to provide means for alleviating (avoiding) entrapment of contaminants in flip-chip assemblies incorporating an interposed preformed planar structure.

According to the invention, a dissolvable preformed planar structure can be formed similarly to other preformed planar structures described herein (especially those of FIGS. 3-5), and used to assemble a semiconductor die to a substrate in much the same manner as described hereinabove (e.g., with respect to FIG. 3). After assembly, the preformed planar structure is dissolved with a suitable solvent, permitting more effective removal of contaminants trapped during the assembly process.

FIG. 9a is a cross-sectional view of a flip-chip assembly, prior to soldering, similar to that of FIG. 3, but employing a dissolvable preformed planar structure 40'. As in FIG. 3, the flip-chip assembly 30 includes one or more silicon chips 32 (two shown) ultimately mounted in face-to-face relationship to a larger silicon chip or substrate 14. Solder balls 36 are formed on the face 32A of each chip 32, and solder balls 38 are formed on the face 34A of the substrate 34 in corresponding positions. Also as in the assembly of FIG. 3, liquid flux is not applied to the face 34A of the substrate 34, prior to soldering the chips 32 thereto. Nor is liquid flux required to be applied to the faces 32A of the chips 32. Prior to soldering the chips 32 to the substrate 34, the preformed planar structure 40', is interposed between the chips 32 and the substrate 34.

As in the assembly of FIG. 3, the preformed planar structure 40' is dipped (not shown) in liquid flux, so that it adheres within the bore of the through holes (or to the surface of the cutouts). This selective application of flux to the solder balls has the same advantages as described hereinabove with respect to FIG. 3 and can be controlled in the same manner as described hereinabove.

After soldering (application of heat to reflow the solder balls), solder joints form within the through holes (or cutouts), and the preformed planar structure is removed by exposing it to a suitable solvent. The result is shown in FIG. 9b, wherein electrical connections 36A formed by reflow soldering remain after removal of the preformed planar structure 40'. After dissolution of the preformed planar structure 40', the assembly 30 can be cleaned with a suitable solvent to remove any remaining contaminants. Alternatively, the preformed planar structure 40' can be made of a material which is soluble in the solvent used to remove the contaminants, permitting dissolution of the preformed planar structure 40' and removal of the contaminants in a single step. One skilled in the art to which this invention most nearly pertains will understand that the solvent must be chosen in accordance to the particular material of which the interposer (preformed planar structure) is formed.

It will be readily appreciated by one of ordinary skill in the art that the use of a dissolvable preformed planar structure provides all of the assembly advantages of the other preformed planar structures described hereinabove, and has the additional advantage that by removal (dissolution) of the preformed planar structure, any contaminants trapped during assembly can be very effectively eliminated. For example, the interposers of FIGS. 6, 6a, 6b, 7, 7a or 7b could readily be dissolved after the chip is connected to a substrate. Moreover, it is within the scope of this invention that a chip is mounted to an interposer such as that of FIGS. 6, 6a or 6b, then the interposer would be dissolved, leaving a chip with exaggerated raised contacts (e.g., 610) on the surface thereof, to later be mounted to a substrate.

Flexible Preformed Planar Structure

The preformed planar structures (interposers) of commonly-owned parent U.S. Pat. Nos. 5,111,279 and 5,168,346 are generally solid and unyielding, and therefore generally unable to conform to poorly planarized substrates such as printed circuit board substrates. (It is assumed that, typically, the die itself is relatively highly planar.) In such a case of mounting a die to a non-planar substrate surface, the interposer structure could actually "lever" the die away from the substrate, which would lead to less reliable solder bump attachment.

According to the invention, the interposer is formed as a ring like structure, rather than as a solid volumetric structure such as has previously been described (e.g., 40). The various "legs" of the ring are relatively (to the volumetric solid) insubstantial and, hence, are able to skew (deform) themselves to accommodate surface irregularities on the surface of an underlying non-planar substrate.

According to an aspect of the invention, the interposer is formed as a ring-like structure with a gap for extra flexibility.

According to a feature of the invention, the interposer structure is formed of polyimide, alumina, berylia, silicon, silicon dioxide, or the like. Materials of unusually high stiffness can be used, and the resulting interposer structure will still exhibit the necessary flexibility for non-planar substrates.

The present invention recognizes (assumes) that bond pads (solder bumps) will generally be arranged in a linear array just inside the periphery of a die.

In an alternate embodiment of the invention, a ring-like interposer structure is provided with a plurality of notches ("kerfs") so that it will exhibit the necessary flexibility for non-planar substrates. Preferably, the kerfs are disposed between the locations of the through holes. The kerfs may be disposed on the top and/or bottom surfaces of the ring-like interposer.

FIG. 10a shows an embodiment of the ring-like interposer structure 1000a of the present invention formed in a narrow, solid (closed) ring 1002a. Because of the narrow legs of the ring shape 1002a, the interposer 1000a is capable of a certain amount of flexing to conform to an irregular surface. By flexing and conforming, the aforementioned "levering" of the die is reduced or eliminated. As stated hereinabove, the ring-like shape of the interposer assumes a similarly shaped ring-like array of solder bumps on a die and substrate between which the interposer will be assembled. The shape of the ring-like interposer 1000a can be readily adapted to any "outline"-shaped array of bond pads, e.g., triangular, circular, etc.. Through holes are omitted from this view, for illustrative clarity, but would be similar to the through holes of the preformed planar structure of FIG. 3. Additionally, the through holes could be pre-filled, as in FIGS. 6, 6a or 6b.

FIG. 10b shows another embodiment of the ring-like interposer structure 1000b of the present invention. In this embodiment, the interposer 1000b is formed as a ring-like structure 1002b with a gap 1004. As a result of the gap 1004, the ring-like structure 1002b is an "open" ring, and the resultant interposer 1000b is even more compliant than the interposer 1000a of the previous embodiment.

In both embodiments, the outside dimension of the interposer is approximately equal to the outside dimension of the die to be attached, and is provided with through holes (not shown) aligned with solder balls on the die (not shown).

In either embodiment, the ring-like interposer structure is able to accommodate significant non-planarity of an underlying substrate without introducing significant levering effects associated with the solid interposer. The interposer does not serve to render the substrate more planar, but by better conforming to surface irregularities (non-planarities) of the substrate the interposer prevents "amplification" of (transmitting of) the non-planarity as "seen" by the die to be attached to the substrate.

The ring-like interposer structures of the present invention are of great advantage when used in conjunction with substrates which are not surfaces of conic projections. For substrates which have surfaces of conic projections and moderate height, solid (versus ring-like) interposers, especially those of certain polymer materials, can accommodate the simple bend of the surface. For substrates of irregular or "bumpy" surface structure, a solid interposer cannot accommodate the bending in the required three dimensions. In the latter case, the ring-like interposer (e.g., 1000a) of the present invention can be used, because the thin ring-like structure can be bent in three dimensions at the same time without the puckering distortions that would be observed in solid interposers. For substrates of perverse surface structure, the ring-with-gap embodiment (1000b) described hereinabove can follow even more significant surface irregularities and still accomplish the objectives of using an interposer structure between a chip and a substrate (e.g., establishing a prescribed standoff and causing the solder joints to be of predetermined mechanical characteristics).

Through-holes are assumed to be present, but are not shown for the interposers of FIGS. 10a and 10b, in order to reduce illustrative clutter. Any of the techniques described hereinabove for the formation of interposers with through holes, solder bumps, liquid flux, etc., may be used in conjunction with a ring-shaped interposer.

In order to better accommodate irregularities in the surface of a substrate, it is possible to "kerf" the interposer in much the same manner as woodworkers are known to "kerf" wood strips to facilitate their being bent into smoothly curved shapes. "Kerfing" is a technique of cutting a series of grooves (notches) in a structural member perpendicular to an intended direction of bending. The grooves effectively "weaken" the member in the direction of bending without significantly altering its stiffness perpendicular to the bend. Such "kerfing" of preformed planar structures is described hereinbelow with respect to FIGS. 10c-10f.

FIG. 10c shows an enlarged top view of a portion of a leg 1002 of a "kerfed" ring-shaped interposer, applicable to the ring-shaped interposers 1000a and 1000b described hereinabove with respect to FIGS. 10a and 10b, respectively. Through-holes 1010 through the leg 1002 of the interposer permit alignment and controlled formation of solder bump contacts as described hereinabove. Slots or grooves (kerfs) 1012 cut (kerfed) in the top surface (as depicted) of the leg 1002 facilitate bending of the leg 1002 in the vertical direction (out of the page, as depicted). FIG. 10d is a side view of the kerfed leg 1002 of FIG. 10c. Evidently, the kerfs 1012 will allow the interposer 1002 to bend in one axis, but will not significantly reduce the stiffness of the interposer in another axis (e.g., out of the page, as shown in FIG. 10d). In the embodiment of FIGS. 10a and 10d, the interposer is kerfed on one surface only, with the kerfs being disposed between the through holes.

Alternatively, the leg 1002 can be kerfed on both its upper and lower surfaces, as shown in FIGS. 10e and 10f, depicting top and side views, respectively. In this case, slots 1012a are also cut in the lower surface of the leg 1002. FIGS. 10e and 10f show the upper slots 1012 and lower slots 1012a in an alternating pattern, but it will be readily apparent to one of ordinary skill in the art that the slots 1012 and 1012a may also be disposed opposite one another. The depth and placement of the slots 1012 and 1012a control the degree of flexibility of the leg 1002. However, when providing deep slots, increased flexibility comes at the cost of the overall strength of the interposer, i.e., deep slots can significantly weaken the interposer.

It is within the spirit and scope of the present invention to combine the techniques described hereinabove with the ring-shaped flexible interposer. For example, in a manner similar to that described with respect to FIG. 3, the flexible interposers can be dipped in liquid flux, etc.. Further, the flexible interposer can be made of a dissolvable material and removed after soldering as described hereinabove with respect to FIGS. 9a and 9b.

Interposer with Integral Probe Fingers

The preformed planar structures (interposers) of commonly-owned, parent U.S. Pat. Nos. 5,111,279 and 5,168,346 are provided with through holes or peripheral cutouts which allow the solder balls of a chip (located on one side of the interposer) to fuse with the solder balls of a substrate (located on the other side of the interposer). When using the through hole embodiment, the resulting solder joints are contained completely within the through holes, and lack controllability and observability. Any attempted probing of the solder joint connections, for whatever intended purpose, is similarly obviated by the fact that the solder joints are hidden within the preformed planar structure. Even in the peripheral cutout embodiment of the interposer, where the solder joints are partially exposed at the periphery of the preformed planar structure, it would be difficult, at best, to probe the joints, due to their location between a chip and a substrate, as well as their small dimensions. Even in the case of a using an interposer such as shown in FIG. 7b, where there is a rectangular array of bond pads (solder bump contacts) between a die and a substrate, probing of "inner" bumps would be virtually impossible, even if a dissolvable interposer (preformed planar structure) were used. Further, any mechanical probing of the surface of a die can cause damage to the die.

According to the invention, a plurality of probe fingers are incorporated into the interposer. For at least a portion, preferably all, of the through holes in the interposer, one end of a probe finger extends to within the through hole, so that when the solder joint between the two solder bumps is made, the probe finger is completely immersed in solder and makes excellent electrical contact with the solder joint. The probe finger extends laterally through the interposer, at least to the peripheral edge of the interposer, and preferably beyond the edge of the interposer. In this manner, making electrical contact to the solder bumps (and solder joints) is accomplished via the probe fingers integral with the interposer.

It is well known in the art that high-speed digital signals propagated in interconnection traces often require a terminating resistor or network. The presence of a preformed planar structure with integral probe fingers provides an opportunity to provide termination components as an integral part of the preformed planar structure. According to this aspect of the invention, passive and/or active circuit elements can be disposed on or within the preformed planar structure. The conductive material of the probe extending from the probe finger into the body of the preformed planar structure can then be used to make contact between a pad on a substrate and/or semiconductor die and the circuit element(s). For example, a termination resistor can be provided for a digital signal by providing a resistive element with the body of a preformed planar structure. In the manner described above, a first probe finger extends into a hole in the preformed planar structure through which a connection to the digital signal is made. The extension of the first probe finger (i.e., the conductive trace extending from the first probe finger into the body of the preformed planar structure) extends into and makes contact with the resistive element. A second probe finger extends into another hole in the preformed planar structure through which a connection to electrical ground is made. The extension of this second probe finger extends into another point in the resistive element. When solder ball contacts are formed through the preformed planar structure, the first and second probe fingers intercept the digital signal and electrical ground, respectively, and provide resistive termination of the digital signal by means of their respective connections to the resistive element.

It will be immediately appreciated by one of ordinary skill in the art that the body of a preformed planar structure of this type can be made of an electrically insulating material. The resistive elements can be formed by depositing or otherwise disposing a resistive material on the surface of or within the body of the preformed planar structure. Entire termination networks for large numbers of digital signals can be formed in this manner. As will be readily appreciated by one of ordinary skill in the art, materials suitable for use in preformed planar structures of this type include: Silicon, Silicon Dioxide, Alumina, Zirconia, and Polyimide.

One of ordinary skill in the art will also readily understand that the circuit elements on or in the preformed planar structure need not necessarily be resistive, and need not be passive components. Other circuit elements which can also be disposed in or on the preformed planar structure include: fixed and variable resistors, fixed and variable capacitors, transistors, particularly thin-film transistors, (including, but not limited to: bipolar, SCR, TRIAC, MOS, CMOS, JFET, etc.), inductors, diodes, zener diodes, tunnel diodes, and varistors.

FIG. 11a is a cross-sectional view of a portion of an embodiment of an interposer with integral probe fingers. An assembly 1100 of a die 1110 to a substrate 1130 includes an interposer 1120 with probe fingers 1125a and 1125b. The die 1110 has a solder ball contact 1140a disposed on a lower face 1110a thereof. The lower face 1110a is positioned above an upper face 1120a of the interposer 1120 such that the solder ball contact 1140a extends into a through hole 1150 in the interposer. The substrate 1130 has a matching solder ball contact 1140b disposed on a surface 1130a thereof. The surface 1130a of the substrate 1130 is positioned below a lower face 1120b of the interposer, such that the solder ball contact 1140b extends into the opposite end of the through hole 1150. A first probe finger 1125a within the interposer 1120 extends into the through hole 1150. Another end of the probe finger 1125a (i.e., the extension of the probe finger) makes contact with an electronic circuit element 1127a. A second probe finger 1125b extends into the same hole 1150, leaving a space between the first probe finger 1125a and the second probe finger 1125b through which an electrical contact may be formed in the through hole 1150. The extension of the second probe finger 1125b contacts a second electronic circuit element 1127b and extends further into the body of the preformed planar structure 1120. The assembly 1100 is shown prior to reflow soldering. After reflow soldering, the two solder ball contacts 1140a and 1140b will fuse into a single solder joint structure, electrically connecting the die to the substrate. Moreover, the portion of the probe finger extending into the through hole in which the solder joint is formed will be in intimate mechanical and electrical connection with the resulting solder joint. Although two probe fingers are shown, any number of probe fingers (including only one) can be extended in similar fashion into any number of respective through holes in any of the interposers described hereinabove or hereinbelow.

The ability to provide a separate means (i.e., probe finger) by which contact can be made to a solder bump contact provides for several novel connection schemes. At the simplest level, selected solder bumps can be connected to one another, without increasing the complexity of either of the solder bump substrates. This permits some increase in complexity of the total circuit without significantly increasing the design or construction complexity of the semiconductor die. It also provides a means by which connections in a preformed planar structure can be effectively "programmed".

It is known in the art to selectively fill holes in a substrate with conductive material. By providing a set of "programmable" connections, each of which will only be made when a particular hole is filled (thereby bridging probe fingers extending into the hole) it is possible to "program" the connections of a preformed planar substrate either prior to assembly or as a part of the assembly process.

Prior to assembly, conductive material can be deposited in selected holes of a programmable interposer (preformed planar structure). The conductive material selects specific connections to be made. Consider the case of a semiconductor die with a programmable clock generator (e.g., programmable frequency multiplier). By selecting a specific pattern of interposer connections to create a specific pattern of logic 1's s and 0's at a set of programming input pads of the die, the clock frequency can be selected by providing an interposer with the right combination of connections. It will be immediately understood by one of ordinary skill in the art that this technique can also be used to provide any pattern of inputs to the pads of a semiconductor die. It will also be understood by one of ordinary skill that this interposer programming technique can be used to re-route connections between the die and the substrate.

Another approach to programming an interposer of this type is to provide the programming connections at the time of assembly. This can be accomplished by providing solder bumps at selected locations on the die and substrate specifically for the purpose of programming connections of the interposer. These "programming" bumps on the die and substrate can be either functional (i.e., connect to another point in the die or substrate) or non-functional (i.e., serve no other purpose than to form the connection in the interposer.)

This technique can further be extended to include passive and/or active electronic components within the interposer, as described hereinabove with respect to FIG. 11a.

A "programmable" interposer of this type is depicted in cross-section in FIG. 11b, wherein an assembly 1100a similar to that shown as 1100 in GIG. 11a has two solder bump contacts 1142a and 1142b formed in respective through-holes 1150a and 1150b, of an interposer 1120. The solder bump contacts 1142a and 1142b are joined by a conductive lead 1125c within the interposer 1120. The solder bump contact 1142a makes further contact with a conductive lead 1125d extending into the through-hole 1150a, thereby electrically connecting to both solder bump contacts 1142a and 1142b. Another conductive lead 1125e extends into the through-hole 1150b, also forming a connection with the solder bump contact 1142b. An opposite end of the conductive lead 1125e extends into a third through-hole 1150c. Another conductive lead 1125f also extends into the third through-hole 1150c. However, no solder bump contact is formed in the through hole 1150c, and therefore no electrical connection is formed between the conductive lead 1125e and 1125f. The conductive lead 1125c makes a connection with an embedded electronic element 1127c (e.g., resistor, capacitor, . . . ). The conductive lead 1125e makes a connection with another embedded electronic element 1127d.

As in FIG. 11a, the die 1110 is positioned with its lower face 1110a above the upper face 1120a of the interposer and the substrate is positioned with its upper face 1130a under the lower face 1120b of the interposer. The solder bump contacts 1142a and 1142b in through-holes 1150a and 1150b, respectively, are shown after reflow soldering. The solder bump contacts capture and are electrically connected by the conductive lead 1125c which extends into the through holes 1150a and 1150b. In this case (i.e., for the purpose of connecting two solder joints together), it is not necessary that the probe finger extend to without the peripheral edge of the interposer, although that may be desired and implemented.

Although the arrangement of FIG. 11b is described as a "programmable" interposer, it will be understood by those of ordinary skill in the art that the interposer-based probe finger techniques can be used without selectively filling (programming) holes of the interposer. That is, the interposer can be used to provide a specific fixed set of connections between a die, a substrate and, optionally, one or more passive and/or active electronic components.

FIG. 11c shows the concept of interposer programming schematically. A set of through-holes 1155a, 1155b, 1156a, 1156b, 1156c, 1156d, 1157a and 1157b in a programmable interposer are indicated as dashed circles in the Figure. Two of the through-holes 1155a and 1155b, correspond to power and ground solder bump connections, respectively, between a semiconductor die and a substrate. Two other through-holes 1157a and 1157b correspond to solder bump connections to two respective pads of a semiconductor die for which the interposer provides programmability. Four other through holes 1156a, 1156b, 1156c, and 1156d are programming holes which, when appropriately filled with conductive material, establish a connection between the "programmable" through holes 1157a and 1157b and one of two logic-level signals. A first conductive lead 1126a has probe fingers extending into two of the programming holes 1156a and 1156c. Conductive lead 1126a also connects to one end of an embedded resistor 1127d. The resistor 1127b can be a thin or thick-film resistor on the surface of the substrate, or can be embedded within the body of the substrate. The opposite end of the resistor 1127b connects to a second conductive lead 1126b which has a probe finger extending into the "power" through-hole 1155a (i.e., the hole through which solder bump contact establishes a power supply connection between a semiconductor die and a substrate). A third conductive lead 1126c has probe fingers extending into programming holes 1156b and 1156d, and into the "ground" through-hole 1155b (i.e., the hole through which a solder bump contact establishes a ground connection between a semiconductor die and a substrate). A fourth conductive lead 1126d has probe fingers extending into the programmable hole 1157a and into two of the programming holes 1156a and 1156b. A fifth conductive lead has probe fingers extending into the programmable hole 1157b and into the two remaining programming holes 1156c and 1156d.

Programming of a logic level at the programmable hole 1157a is accomplished by filling either programming hole 1156a or programming hole 1156b with conductive material. If the programming hole 1156a is filled with conductive material, then the probe finger extending into the programmable hole 1157a is resistively connected to the power supply (via resistor 1127d to the "power" through-hole 1155a) thereby establishing a logic "1" level at the programmable through-hole 1157a. Alternatively, if the programming hole 1156b is filled with conductive material, then the probe finger extending into the programmable hole 1157a is directly connected to ground (via conductive lead 1126c to the "ground" through-hole 1155b) thereby establishing a logic "0" level at the programmable through-hole 1157a. The resistor 1127d provides protection against inadvertently filling both holes (1156a and 1156b) with conductive material. It will be readily appreciated by one of ordinary skill in the art that the appropriate programming hole can be filled either by pre-depositing conductive material in the hole prior to assembly or by providing a "programming" solder bump contact at the location of the programming hole.

Programming of a logic level at the programmable hole 1157a is accomplished similarly. If the programming hole 1156c is filled with conductive material, then the probe finger extending into the programmable hole 1157b is resistively connected to the power supply (via resistor 1127d to the "power" through-hole 1155a) thereby establishing a logic "1" level at the programmable through-hole 1157b. Alternatively, if the programming hole 1156d is filled with conductive material, then the probe finger extending into the programmable hole 1157b is directly connected to ground (via conductive lead 1126c to the "ground" through-hole 1155b) thereby establishing a logic "0" level at the programmable through-hole 1157b. As before, the resistor 1127d provides protection against inadvertently filling both holes (1156c and 1156d) with conductive material. The appropriate programming hole can be filled either by pre-depositing conductive material in the hole prior to assembly or by providing a "programming" solder bump contact at the location of the programming hole.

Assuming that a die and a substrate (which may be another die) are of approximately the same size (e.g., two flip-chipped dies), it is very difficult, using conventional means, to join them together into any useful configuration, because there is no room to make conventional (e.g., wire bond) electrical contact from the flipped-together dies to a package or circuit board. The use of a traced interposer with the probe fingers extended out to external leads or contacts solves this problem. Two dies are joined face-to-face with a traced interposer disposed between the two dies. The traces of the interposer extend outward from the solder bump area, for connecting the two flip-chipped dies to another substrate.

It will be readily appreciated by one of ordinary skill in the art that the use of the interposer with finger traces can facilitate face-to-face (flip-chipped) connections between dies of virtually any relative sizes. It will further be appreciated that it is possible for the interposer finger traces to extend out from the edges of the die stack as conductive leads or contacts to become part of any of a large variety of packages and substrates, including: printed circuit boards, plastic packages, and M-Quad packages, to name but a few examples.

FIG. 11d illustrates the case wherein two similar-size dies 1170a and 1170b are flip-chip assembled with a "traced" interposer (e.g., 1120, FIGS. 11a and 11b) disposed between them to form an assembly 1155. The traces within the interposer (not shown, compare FIGS. 11a,b) are extended outside of the interposer to form leads 1180 by which the assembly 1155 can be mounted and electrically connected to a substrate 1160.

It will be readily appreciated by one of ordinary skill in the art that a single interposer can be sized to extend across several dies, and need not be restricted to assemblies of two dies, or a die and a substrate. Since the interposer can extend across the area of several dies and/or substrates, evidently a substantial improvement in circuit density can be realized, with the associated aforementioned benefits of the use of the present inventive techniques. It should be noted that the interposer technology of the present invention is completely compatible with conventional printed circuit board manufacturing technology.

One of ordinary skill in the art will appreciate that it is possible to create interposers including multiple layers of traces between layers of insulating material. Assuming adequate dielectric characteristics of the insulating layers, it is possible to create wiring structures in the interposer which cross each other without electrically shorting. Also, in a manner similar to that used in multi-layer printed circuit boards, the multiple layers of traces can be interconnected (e.g., by conductive vias), as desired.

According to one aspect of the invention, the probe fingers can become part of a structure which can be used to test the die functionally. A plurality of traces exiting the interposer can be connected to a test system, for testing the die in ways otherwise not possible via the substrate, for example, by providing access to every solder bump contact of the die. Such an arrangement is shown in FIG. 11e.

According to another aspect of the invention, integral termination networks can be provided within the interposer by providing appropriate terminating electronic elements (e.g., resistors) at through-holes corresponding to solder bump contacts requiring termination (e.g., by extending conductive probe fingers which connect to the terminating elements into the holes).

FIG. 11e shows an assembly 1185 including an interposer 1190 to which at least one die (not shown) is mounted. Conductive traces 1194 are disposed within the interposer 1190 (compare FIG. 11a) to respective connection points 1192 (e.g., similar to probe fingers 1125a of FIG. 11a). A ribbon cable 1195, or other suitable interconnection means is electrically connected to the traces 1194, where they exit the peripheral edge of the interposer, thereby forming an electrical interface for external test equipment (not shown).

A set of through-holes (e.g., 1192b) is shown through which it is intended that die-to-substrate solder bump contacts be formed. At least one of these through-holes 1192d, is used to provide a ground connection to the die. A subset 1192e of these through-holes 1192b require termination. For each of the holes in this subset 1192e, a connection to a resistive element 1193 is provided. The opposite end of each resistive element 1193 is connected to ground via the ground through-hole 1192d. Assuming that a set of programmable holes are provided in the interposer 1185, there are programming holes 1192c (compare FIGS. 11a-d.).

According to another aspect of the invention, the probe fingers can be made to create separate interconnections (e.g., via a cable similar to 1195, or other suitable interconnection means) between two-or-more interposer structures (e.g., 1190), thereby permitting increased wiring complexity without increasing the complexity of either the die or the substrate. In other words, the probe finger traces exiting two or more interposers (such as the interposers shown in FIGS. 11d and 11e) can be connected to one another, and in some cases not connected to an underlying substrate at all.

One of ordinary skill in the art will appreciate that any or all of the "programming holes" (e.g., 1192c) can be replaced with embedded electronic switches (e.g., FET's or similar elements) in the interposer to provide for electronic programmability of the interposer rather than the "mechanical" hole-filling technique. This is possible since any type of electronic element, active or passive, can be embedded in the interposer. Embedded memory elements can be provided within the interposer to retain switch programming information and to provide non-volatile storage therefor. Programming of the switches can be accomplished via other through holes, via extended conductive leads of the interposer (e.g., 1180 FIG. 11d.), via a test access "ribbon cable" (e.g., 1195), or by any other suitable means (e.g., fuse blowing, laser trimming, etc.). This could be used to provide a programmable (or re-programmable, in the case of non-volatile memory) cross-point switching matrix embedded within the interposer to provide for many different possible interconnection schemes.

It will be readily appreciated by one of ordinary skill in the art that the "traced" interposers of this invention, with their associated probe finger contacts, may provide electrical contact with the solder bumps on a facing die, if the die and substrate are placed under moderate pressure to force them together, even before the solder bumps are melted together. (This assumes, of course, that the through holes in the interposer are sized to permit the solder ball on the die to contact the probe finger.) This feature would permit preliminary testing of the die and substrate prior to final assembly (i.e., prior to heating to fuse the solder balls to one another).

Additional features, advantages, benefits and possibilities of incorporating active and passive circuit elements into the interposer structure are discussed hereinbelow, following discussion of additional embodiments of the invention.

It is important to note that the interposers of the present invention are not required to be, but preferably are, mechanically strong enough to support the mounting of a facing die, or of a die and a substrate, to another major assembly, without other mechanical reinforcement. The material of the interposer is chosen to give the required mechanical properties. Although an interposer can be made of a single insulating and a single conducting material, benefits can be realized from the use of several different materials which are used as a series of layers to create the interposer.

Adhesive layers, e.g., contact adhesive, thermoplastic, or a thermosetting resin material can be applied to the faces of the interposer. These adhesive layers can be made to adhere strongly to a die, to a substrate material, or even to a PCB. The adhesive layer causes the die (or substrate) to grip the interposer firmly, providing additional mechanical stability.

As reflow-soldered circuits undergo temperature cycling, it is not uncommon for failures to occur at the solder bump structure due to thermally induced flexing. By holding the die to a substrate via an adhesive layer, the flexing that might otherwise break the connection between the die and the substrate is reduced. The adhesive can also be used to keep the substrate/interposer/die stack under compression, which has the benefit of holding together solder bumps which might otherwise be mechanically weakened.

Inert Metal Stoppers retarding electrogalvanic-induced corrosion

The-preformed planar structures of commonly-owned U.S. Pat. Nos. 5,111,279 and 5,168,346 are provided with through holes or peripheral cutouts which allow the solder balls of a chip (located on one side of the interposer) to fuse with the solder balls of a substrate (located on the other side of the interposer).

Solder balls (or bumps) are structures which are used to electrically connect a circuit board or other substrate to a semiconductor chip. These bumps are created in a number of ways, including: electron beam evaporation, high vacuum sputtering, and electroplating. In each process there are variations which make it difficult to maintain the exact composition of the deposited material from run-to-run. As a result, there can be significant differences in the composition of solder bumps on different substrates, even though they may have been formed by a similar process.

Solder bumps are typically fabricated from a variety of metals and their associated alloys. Typical of these metals are: lead, tin and zinc. Unless the solder bump structures on both the substrate and the chip are of exactly the same composition, electro-galvanic corrosion of the resulting solder joints can occur. The subject of electro-galvanic corrosion is discussed, in another context, in U.S. Pat. No. 4,478,915, incorporated by reference herein.

According to the invention, an inert metal film structure is provided in the interposer and physically separates the dissimilar alloys of dissimilar solder bumps, and largely inhibits electro-galvanic corrosion processes that would otherwise be associated with solder joints formed by mixed (dissimilar) solder bumps.

Generally, a thin film (foil) of a noble (inert to galvanic corrosion) metal is incorporated into the holes of the interposer to galvanically isolate the bumps of the chip from those of the substrate. Electrical conductivity between the bumps of the chip and the substrate is maintained by the conductivity of the noble metal. By physically separating the bumps of the chip from the bumps of the substrate, chips having solder bumps formed of various metals (and alloys) can be joined to a substrate, and vice-versa, without concern for galvanic corrosion.

FIG. 12a is a cross-sectional view of an interposer having a noble metal foil disposed in a through hole (one of many shown) to galvanically isolate the material of the die ball bump from the material of the substrate ball bump, while permitting effective electrical connection between the two ball bumps. An assembly 1200 of die/interposer/substrate is illustrated at a stage immediately prior to reflow soldering. The semiconductor device assembly 1200 is formed of a semiconductor die 1210, an interposer (preformed planar structure) 1220, and a substrate 1230. A solder bump contact 1240a is disposed on a bottom surface 1210a of the semiconductor die 1210. A mating solder bump contact 1240b is disposed on a top surface 1230a of the substrate 1230. A through hole 1250 extends through the interposer from its top surface 1220a to its bottom surface 1220b. An inert metal film 1225 (e.g., gold) is embedded in the interposer 1230 such that it extends into and across the through hole 1250, effectively blocking the hole 1250 and dividing it into a top portion 1250a and a bottom portion 1250b. The bottom surface 1210a of the semiconductor die 1210 is positioned in face-to-face alignment with the top surface 1220a of the interposer, such that the solder bump 1240a on the die 1210 extends into the top portion 1250a of the through hole 1250 towards the noble metal film 1225. The top surface 1230a of the substrate 1230 is positioned in face-to-face alignment with the bottom surface 1220b of the interposer, such that the solder bump 1240b on the substrate 1230 extends into the bottom portion 1250b of the through hole 1250 towards the noble metal film 1225.

When heat is ultimately applied to reflow the two solder bump contacts 1240a and 1240b, they will melt and bond to their respective sides of the metal film 1225. Because of the metal film 1225, the two solder bump contacts 1240a and 1240b can not come into direct (physical or chemical) contact with one another, but electrical conductivity therebetween is provided by virtue of the electrical conductivity of the noble metal film 1225. As a result, an electrical and mechanical bond is formed between the solder bump contact 1240a and 1240b without a chemical bond which could cause electrogalvanic corrosion.

It will be readily appreciated by one of ordinary skill in the art, that since the noble metal film 1225 is in electrical contact with the two solder bumps, it could readily be extended to outside the peripheral edge of the interposer in the manner of the probe fingers and traces (see FIGS. 11d and 11e) to provide external access to the solder bump contacts 1240a and 1240b via the interposer, or could be extended within the interposer structure to form electrical connections with other solder bumps (see FIG. 11b). The only significant difference between the arrangement of FIG. 12a and those of FIGS. 11a and 11b is that the noble metal film 1225 physically separates the solder bumps 1240a and 1240b after reflow, while the probe finger 1125a (FIG. 11a) permits the solder bumps (1140a and 1140b) to flow around the end of the probe finger and mix after reflow.

FIG. 12b illustrates the combination of inert metal stoppers (see FIG. 12a) and the techniques described hereinabove with respect to FIGS. 11a-e. An assembly 1200a of die 1212, interposer 1222, and substrate 1232 is illustrated at a stage immediately prior to reflow soldering. Solder bump contacts 1242a and 1242c are disposed on a bottom surface 1212a of the semiconductor die 1212. Mating solder bump contacts 1242b and 1242d are disposed on a top surface 1232a of the substrate 1232. Through holes 1252a and 1252b extend through the interposer from its top surface 1222a to its bottom surface 1222b. Inert metal film stoppers 1225a and 1225b (e.g., gold) are embedded in the interposer 1232 such that they extend into and across the through-holes 1252a and 1252b, respectively, effectively blocking the through-holes 1252a and 1252b and dividing them into top portions 1254a and 1254c, respectively, and bottom portions 1254b and 1254d, respectively. Similar to the arrangement of FIG. 12a, the bottom surface 1212a of the semiconductor die 1212 is positioned in face-to-face alignment with the top surface 1222a of the interposer, such that the solder bumps 1242a and 1242c on the die 1212 extend, respectively, into the top portions 1254a and 1254c of the through holes 1252a and 1252b, respectively, towards the noble metal films 1225a and 1225b. Similarly, the top surface 1232a of the substrate 1232 is positioned in face-to-face alignment with the bottom surface 1222b of the interposer, such that the solder bumps 1242b and 1242d on the substrate 1232 extend into the bottom portions 1254b and 1254d of the through holes 1252a and 1252b, respectively, towards the noble metal films 1225a and 1225b.

When heat is ultimately applied to reflow the solder bump contacts 1242a-d, they will melt and bond to their respective sides of the metal films 1225a and 1225b. The metal films 1225 prevent direct (physical or chemical) contact between the opposing solder bump contacts (1242a,b and 1242c,d), but electrical conductivity therebetween is provided by virtue of the electrical conductivity of the noble metal films 1225a and 1225b.

Unlike the arrangement of FIG. 12a, however, the noble metal film 1225a extends into one end of an embedded electronic element 1227, and noble metal film 1225b extends into another end of the embedded electronic element 1227. If, for example, the embedded electronic element is a resistor, then a resistive connection is formed between conductive bump contacts 1242a,b and conductive bump contacts 1242c,d. Access holes (e.g., 1255) can be provided to permit programmability of the noble metal conductors as described below with respect to FIG. 12c.

Physical programmability of noble-metal film conductors can be accomplished as shown in FIG. 12c, wherein an access hole 1255 in the interposer 1222 exposes a narrowed portion of the noble metal 1225b, permitting it to be cut (e.g., with a laser).

Stepped Interposer Structures

The preformed planar structures (interposers) of commonly-owned U.S. Pat. Nos. 5,111,279 and 5,168,346 are provided with through holes or peripheral cutouts which allow the solder balls of a chip (located on one side of the interposer) to fuse with the solder balls of a substrate (located on the other side of the interposer). The preformed planar structure is disposed entirely between the chip and the substrate. Generally, in use, the interposer is placed on the substrate, and the chip is placed on the interposer. Although the through holes (or peripheral cutouts) provide some self-alignment of the die to the substrate, the process still requires accurate placement of the die.

It is therefore an object of the present invention to provide an interposer that functions to self-align a die over a substrate.

According to the invention, an interposer is formed which has a cavity in its top surface, into which the die is placed. Preferably, the cavity has tapering side walls to facilitate placement of the die into the interposer cavity. Evidently, such an interposer must be larger than the die, since the cavity is approximately the size of the die. Hence, there is a portion of the interposer that is between the die and the substrate, and a portion (including the cavity) that encompasses the die (surrounds the edge of the die). The interposer may be formed as an integral unit (both portions formed as one piece), or the die-encompassing portion can be formed separately and joined to the interposed portion. In the latter case, preferably, indexing keys are formed on the joining faces of the two portions to ensure that they are correctly assembled to one another. The two portions are joined with a suitable adhesive or by ultrasonic bonding.

FIG. 13a shows a portion of a semiconductor device assembly 1300 using a one-piece stepped interposer 1302. The interposer 1302 has a cavity 1310 extending into its upper surface 1302a. The cavity 1310 is sized and shaped to receive and align a semiconductor die 1320. Through holes 1330 (one of many shown) extend from the cavity (underneath the die 1320) through the bottom surface 1302b of the interposer, to provide further alignment of the die 1320 (by its solder bump contacts, not shown) and controlled formation of solder bump electrical connections.

In use, a die is placed in the cavity of the interposer, and both are assembled to a substrate. The interposed portion of the interposer (i.e., the portion underneath the die) can be provided with unfilled through holes (compare FIG. 3), with filled through holes (compare FIGS. 6, 6a, 6b), the die can be encapsulated (compare FIG. 6c), the interposer can function as a pitch spreader/adaptor (compare FIGS. 7, 7a, 7b, 8, 8a), the interposer may be dissolvable (compare FIGS. 9a, 9b), the interposer can be formed as a ring-like structure (compare FIGS. 10a-f, in which case a portion of the cavity would extend completely through the interposer), and the interposer can be provided with probe fingers (compare FIGS. 11a-d) or anti-galvanic foils (compare FIG. 12).

The interposer can also be designed to accommodate more than one semiconductor die, as shown in FIG. 13b.

FIG. 13b is a cutaway view of a portion of an arrangement 1300a using a two-piece stepped interposer designed to accommodate two semiconductor dies. The interposer is formed of a planar base portion 1304 and a top portion 1306. The top portion 1306 has tapered openings 1310a and 1310b sized and shaped to receive and align semiconductor dies 1320a and 1320b, respectively. The top portion 1306 is assembled to a top surface 1304b of the base portion 1304 such that when placed in the openings 1310a and 1310b, the dies 1320a and 1320b, respectively, rest on the top surface 1304b of the base portion. Through holes 1330 and 1330a under the dies 1320a and 1320b extend through the base portion 1304 to its bottom surface 1304a. Solder bump contacts on the dies (not shown) extend into these holes 1330 and 1330a to form electrical connections with mating contacts on a substrate (not shown) to which the arrangement 1300a is ultimately assembled.

Clear or Translucent Interposer

The preformed planar structures (interposers) of commonly-owned parent U.S. Pat. Nos. 5,111,279 and 5,168,346 generally disclose an interposer aligning a die in the x, y and z axes above a substrate. One of ordinary skill in the art, however, will readily appreciate that an element other than a semiconductor die (chip) can be x-y-z aligned above a die (or substrate) in the same or similar manner.

In certain integrated circuit applications, such as EPROMs, optical sensors, and the like, it is often necessary to locate an optical element, such as a lens or a window, above a die. In some of these cases, it is important to maintain a predetermined spacing (z-axis) between the element and the die, as well as locating the element (x and y axes) directly above the die.

With certain mechanical arrangements, wherein there is an air space between the die and the element disposed above the die, condensation may form on an inner surface of the (optical) element.

It is therefore an object of the invention to provide a technique for mounting an element, such as an optical element, above a die, with predetermined x-y-z spacing and orientation, while avoiding condensation on an inner surface of the element.

According to the invention, a clear (transparent) interposer (preformed planar structure) is disposed in and fills a space between a die and an element disposed above the die

According to an aspect of the invention, the interposer may be clear, or it may be colored. If colored, the interposer may exhibit the additional functionality of a color filter, such as for camera applications. Where image coherence is not required, as in a UV-erasable EPROM application, a translucent interposer may be used to great advantage.

According to an aspect of the invention, the interposer may have certain optical characteristics, such as:

having binary optics formed on a surface of the interposer;

having darkened areas to prohibit light from impacting selected areas (e.g., logic elements) of the underlying die, while allowing light to impact other areas (e.g., photosensitive elements) on the underlying die.

According to an aspect of the invention, the interposer can be provided with embedded conductors terminating on the die-facing of the interposer and extending to an external surface of the interposer. In this manner, electrical connections to the die can be made through the interposer.

According to an aspect of the invention, the interposer may have its die-facing surface formed with indentations or through holes (or cutouts) arranged in a pattern matching elevated features (e.g., solder balls or other "relief" features) on the die, for ensuring precise x-y alignment with the die.

According to an aspect of the invention, the interposer may have its element-facing surface formed with a lip, or the like, for ensuring precise x-y alignment with one or more edges of the element.

According to an aspect of the invention, the surface of the interposer facing away from the die can be molded as a lens, or can be formed with integral binary optics.

Examples of materials for forming a clear or translucent interposer include: quartz, silica, polyvinylchloride (pvc), polyvinylcarbonates, and lucite.

FIG. 14a is a cross-sectional view of an arrangement 1400a of a semiconductor die 1410a and a light transmissive window 1430a, separated by a substantially clear interposer 1420a. The window 1430a is built into a semiconductor device package 1440a and is, for example, a quartz window in a ceramic package for an EPROM. The interposer 1420a is "registered" to the semiconductor die by means of "lips" 1421a on the interposer 1420a which capture the edges of the die 1410a (compare the interposer with cavity of FIG. 13a) The interposer completely fills the gap between the window 1430a and the die 1410a, thereby preventing the formation of condensation therein. If image coherence is not required through the interposer (e.g., as in a UV-erasable EPROM application), then the interposer can be translucent. Otherwise, the interposer should be transparent.

FIG. 14b is a cross-sectional view of portion of a semiconductor device assembly 1400b including a lens 1430b. In this case, a clear or colored transparent interposer 1420b is used to accurately position an optical element 1430b (lens shown) relative to a semiconductor die 1410b. The interposer 1420b has a planar section 1422b and a raised section 1421b (compare the interposer with cavity of FIG. 13a). The raised section 1421b captures and positions the optical element 1430b relative to the die 1410b. The planar section 1422b is clear and permits transmission of a coherent image therethrough. In the case of a colored transparent material, the planar section 1422b serves as a color filter for e.g., a camera application. Alternatively, the planar section 1422b can be omitted, leaving only the raised section 1421b in a ring-shaped configuration (similar to that described hereinabove with respect to FIGS. 10a-f). In the case of the ring-shaped interposer, it may not be necessary (or even desirable) to use a clear or translucent material, since the interposer could be used as a light blocking wall between an optically sensitive area and a logic area of the die 1410b.

FIG. 14c illustrates another use of a clear or translucent interposer. A semiconductor die 1410c has an optically sensitive area 1440 and a logic area 1450. As is well known to those of ordinary skill in the art, semiconductor devices are generally light sensitive, and in order to prevent mis-operation of logic elements (e.g., 1450), it is often necessary to shield them from exposure to light. However, on the semiconductor die 1410c which has both an optically sensitive area 1440 and a logic area 1450, a transparent or translucent interposer 1420c with a darkened (light blocking) area 1460 is used to shield the logic 1450 from extraneous exposure to light. In a camera application, the non-light-blocking area of the interposer could be a colored transparent material to act as a colored filter, or a clear in an application which does not require color selectivity. Other applications which do not require image coherence through the interposer 1420c can make use of a translucent material.

The surface of the interposer 1420c can also be given other optical characteristics, such as binary optic features 1430c formed on the surface of the interposer. (In order for this to be useful, the interposer must be made of a clear or colored substantially transparent material.

The interposers 1420b and 1420c described with respect to FIGS. 14b and 14c, respectively, can be aligned to the semiconductor die by means of indentations or raised features (not shown) to mate with corresponding raised features (e.g., solder bumps) or indentations, respectively, on the semiconductor die.

FIG. 14d illustrates a semiconductor device assembly 1400d which combines the clear/colored/translucent interposers with the traced interposer techniques described hereinabove with respect to FIGS. 11a-d. In this case, an optically sensitive semiconductor die 1410d has one or more solder bump contacts 1480 disposed on the top surface thereof. A clear, colored, or translucent interposer 1420d is aligned over the semiconductor die 1410d by means of one or more indentations 1490 in the interposer which mate with the one or more solder bumps 1480. In a manner similar to that described with respect to FIG. 11a, conductive traces 1470 extends into the indentations and, after reflow soldering, make contact with the solder bump contacts 1480. The traces can be used to connect to other solder bumps, to connect to (or form) pins of a package body, etc..

Use of Preformed Planar structure in multi-chin structures

The preformed planar structures (interposers) of commonly-owned parent U.S. Pat. Nos. 5,111,279 and 5,168,346 and method of assembling flip-chip type assemblies contemplates, for example, mounting two dies to a substrate (see e.g., FIG. 3). The use interposers can also be extended to more elaborate assemblies of chips.

Semiconductor technology has shown a general trend towards dramatic increases in integrated circuit speed and density. Both of these trends are fueled by a general reduction in device (active element) geometries. As semiconductor devices become smaller, the distances between them on a semiconductor die become smaller, and parasitics (such as parasitic capacitances) and switching currents become smaller. In technologies such as CMOS, where overall current draw and switching speed characteristics are dominated by the effects of parasitics, the result is a reduction in total power consumption at the same time as switching speed is improved. Overall speed is further improved by the reduction in signal propagation time between active devices (e.g., transistors) resulting from the shorter distances involved. In today's high speed integrated circuitry based on sub-micron geometries, delays in the tens or hundreds of picoseconds can be appreciable.

Typically, integrated circuit dies (chips, or semiconductor dies) are diced (cut apart, or singulated) from a semiconductor wafer and are assembled into integrated circuit packages which have pins, leads, solder (ball) bumps, or conductive pads by which electrical connections may be made from external systems to the integrated circuit chip. These packages are then typically applied to circuit board assemblies comprising systems of interconnected integrated circuit chips and other electronic components.

The aforementioned dramatic improvements in integrated circuit speed and density have placed new demands on integrated circuit assemblies, both at the chip and circuit board levels. Without attendant improvements in these areas, much of the benefit of high device speed is lost. Wiring propagation delays and transmission line effects, in integrated circuit packages and on circuit board assemblies, which were once negligible are now significant factors in the overall performance of systems based on high-speed integrated circuitry. In order to achieve the potential higher system-level performance opportunities afforded by the new high-density technologies, it is necessary to reduce the amount of signal propagation time between integrated circuits.

Another significant factor in achieving high system-level performance is signal drive capability. Longer signal paths are susceptible to noise (cross-talk, etc.) and require low-impedance, high-current drive circuits on the integrated circuit chips (dies). Such circuits tend to occupy large portions of the die area (either reducing the area available for other circuitry or increasing the overall die size), and can introduce significant delays of their own. Clearly, shorter signal paths and their attendant low signal drive current requirements are desirable to achieve high performance.

Generally, the use of flip-chip arrangements facilitates high-density, high-performance packaging. Examples of various prior-art "flip-chip" constructions are found in U.S. Pat. Nos. 3,388,301; 4,545,610; 4,190,855; 4,811,082; 4,926,241; 5,039,628; 4,970,575; 3,871,014; and in Japanese Patent Numbers 61-142750; 61-145838; 57-210638; and in "Wafer-Chip Assembly for Large-Scale Integration", IEEE Trans. Elec. Dev., Vol. ED-15, No. 9, September 1968.

Inasmuch as the interposer structures disclosed herein are useful for flipping two or more semiconductor dies together, the interposer structures are useful in building stacked arrangements of semiconductor dies, including one die overlapping two other dies in a "brickwork" type of configuration. For more than two tiers of stacked dies, dies connecting to a layer of dies both above and below a tier may be provided with contact pads (and/or bumps) on both sides of the die. As is somewhat evident, the stacking of dies has the potential for increasing the number of input and output (I/O) connections that are present in a stacked assembly of a given area (footprint), versus a single die, based on similar spacing requirements of contact pads.

It is an object of this invention to provide an improved technique for forming multi-chip semiconductor assemblies, especially stacked multi-chip assemblies.

According to the invention, flip-chip assemblies are formed using a preformed planar structure interposed between the chip(s) and the substrate. This includes chip-on-chip, chips-on-chip, small die on large die, more than one small die on a larger die or substrate, "brickwork" and multi-tier arrangements. The preformed planar structure may have either through holes or peripheral cutouts. Unless otherwise specified, the terms "chip", "die" and substrate are generally used interchangeably.

Chip-on-chip assemblies have been described hereinabove, and can generally advantageously employ any of the different types of interposers. Some arrangements have been described as assemblies of a semiconductor die to a substrate with an interposer (preformed planar structure) between, but it will be readily appreciated by one of ordinary skill in the art that a semiconductor die may be employed as a substrate, thereby effectively forming multi-chip assemblies. In particular, FIG. 11c described a face-to-face flip-chip assembly with an intervening interposer. Conductive traces embedded in the interposer were used as "pins" of the flip-chip assembly for further assembly to another chip or substrate.

FIG. 15a shows a small-die-on-large-die flip chip arrangement (assembly) 1500a, wherein a large die 1510a is used as a substrate. An interposer 1520a, such as those described hereinabove is disposed between the large die 1510a and a smaller die 1530a. In this case, the interposer 1520 is approximately the same size as the smaller die. The interposer 1520a can be solid (rectangular) or ring-shaped (see FIGS. 10a-f), and may have embedded conductive traces (see FIGS. 11a-d, 12).

FIG. 15b shows a multi-tier flip-chip arrangement (assembly) 1500b, with interposers disposed between each of the "tiers". Two "base-level" (bottom tier) dies 1510b are bridged by a second-level die 1530b. An interposer 1520b separates the base-level dies 1510b and the second-level die 1530b. A pair of third-level (top tier) dies 1531b are "flipped" on the second-level die 1530b by means of a pair of interposers 1521b, thereby forming a three-tier flip-chip structure with all advantages described hereinabove with respect to interposers in semiconductor device assemblies. As before, any of the other features of interposer described hereinabove can be employed in the multi-tiered flip-chip configuration, e.g., the interposers can be either solid or ring-shaped, and can have embedded conductors.

According to a feature of the invention, liquid flux is applied to the preformed planar structure in order that the flux is selectively applied to the solder balls (pads) on the chip(s) and the substrate.

According to an aspect of the invention, the preformed planar structure has a planar core and opposing planar faces. The core is formed of thermosetting organic resin, such as polyimide, or non-organic material such as alumina, polished sapphire, beryllium oxide, aluminum or aluminum nitride. The planar faces of the preformed planar structure are formed of thermoplastic resin or thermosetting material, such as polyacetal, epoxy (epoxide resins) or polystyrene.

According to an aspect of the invention, the preformed planar structure has a thickness of 5-50 microns, preferably on the order of 20-30 microns.

FIG. 15c is a cutaway view of an assembly of two semiconductor dies 1510c and 1530c separated by an interposer 1520c. The first semiconductor die 1510c has a solder bump contact 1560 disposed on a surface thereof. Similarly, the second semiconductor die 1530c has a matching solder bump contact 1550 disposed on a surface thereof. The two dies 1510c and 1530c are arranged in a face to face configuration such that the two solder bump contacts 1560 and 1550 are aligned. The interposer 1520c is disposed between the two dies 1510c and 1530c such that the solder bump contacts 1560 and 1550 extend into opposite ends of a through hole 1540 in the interposer. Upper and lower faces 1524c and 1522c of the interposer 1520c are made of a thermoplastic resin or thermosetting material, such as those described above. Upon application of heat, the faces 1522c and 1524c of the interposer 1520c form adhesive bonds with the two dies 1530c and 1510c, respectively. At the same time, the two solder bump contacts 1550 and 1560 melt and fuse to form an electrical connection between the dies 1510c and 1530c. The interposer 1520c has a thickness "d" between 5 and 50 microns, preferably 20-30 microns.

According to an aspect of the invention, it is possible to provide repairable flip-chip assemblies by using interposers with through holes filled with a resilient conductive plastic material (e.g., epoxy or other suitable plastic conductor). Such an arrangement is illustrated in cross-section in FIG. 16. A flip chip assembly 1600 includes a substrate 1630, a die 1620, and an interposer 1610.

It is known in the art and is relatively simple to screen print or otherwise apply conductive material (e.g., conductive epoxy or silicon materials) to fill holes in a substrate, e.g., a preformed planar substrate (interposer). This is especially easy to accomplish if the holes are to be filled flush to the surface of the interposer. Using known techniques, however, it is somewhat difficult to create conductive structures on a die which are capable of contacting flush structures on an interposer reliably.

Without disposing ball contacts on the pads of a die, access to the electrical signals of a typical semiconductor die is accomplished by means of conductive pads accessible through openings in a top passivation layer (i.e., on an exposed surface) of the die. The pads are typically recessed below the surface of the passivation layer. One approach to making contact with recessed pads is shown and described hereinabove with respect to FIG. 6b, wherein conductive structures extending out of an interposer are provided. These raised structures can extend into and contact the recessed pads of the die.

According to the invention, a simpler approach is to build up raised (e.g., columnar) structures on top of the conductive pads of the die. The columnar structures are formed of a robust conductor (e.g., copper or aluminum). The columnar structures protrude above the surface of the die (above the surface of the top passivation layer). Similar, matching columnar structures are formed on top of a substrate to which the die is to be connected. (The substrate can be another die.) An interposer is provided with plastic conductive "plugs" filling through-holes in the interposer at positions corresponding to the locations of the columnar structures on the die and substrate. The conductive plugs are substantially flush with the surface of the interposer. The die and substrate are placed in a face-to-face orientation with corresponding columnar structures on each in alignment with one another. The interposer is positioned between the die and substrate such that each plastic conductive plug is positioned between and aligned with a corresponding pair of columnar structures (one on the die and one on the substrate). The die, substrate, and interposer are then place under pressure in a suitable fixture (e.g., by closing a compressing cover or clip, not shown) such that the columnar structures come into intimate contact with the conductive plugs under force. Because of the resilient nature of the plastic material filling the through holes in the interposer, it is capable of deforming slightly and conforms to the shape of the contacting surfaces of the columnar structures, forming robust electrical connections therewith. If, at some point in the future, it becomes necessary to replace the die (or the substrate, if it is another die) the pressure is removed (e.g., by opening a compressing cover), and the die can be removed and replaced with another, similar die. This allows for replacement of defective dies (or substrates, or interposers for that matter), for example, in stacked or tiered arrangements.

An stacked arrangement of dies (or a die and a substrate) is shown in cross-section in FIG. 16. A semiconductor device assembly 1600 comprises an interposer 1610, a die 1620, and a substrate 1630, held together under a compressing force. (The compressing force is indicated in the figure by downward pointing arrows ↓↓↓↓. One skilled in the art to which this invention pertains will understand that any suitable mechanism for applying such force without damaging the die can be employed.) A columnar (raised bump) structure 1640a is built up on a bond pad 1645 of the die 1620 and a mating columnar structure 1640b is formed on the surface of the substrate 1630. The die and substrate are positioned on opposite sides of a through-hole 1615 through the interposer 1610. A plastic conductive plug 1650 fills the through-hole 1615, and is held in compression contact with the two columnar structure 1640a and 1640b by the compressing force. As shown in the Figure, the conductive plug 1650 deforms slightly at the points of contact with the two conductive columnar structures 1640a and 1640b, forming a low-resistance electrical connection through the conductive plug.

It will readily be appreciated by one of ordinary skill in the art that this technique can be used in combination with the other techniques described hereinabove (e.g., probe fingers, programmable connections, embedded electronic components, etc.).

According to another aspect of the invention, gas (gaseous, e.g., air) cooling of a flip-chip assembly can be facilitated by embossing a set of cooling channels on the surface of an interposer to permit cooling gas to flow between the interposer and a die (or substrate). The cooling gas is directed at the assembly substantially in alignment with the cooling channels.

FIG. 17a is a side view of a flip-chip assembly 1700a using an embossed interposer 1710a. A die 1720 is mounted to a substrate 1730 via the interposer 1710a, by any suitable means, such as those described hereinabove with respect to FIGS. 1-16. The interposer 1710a has cooling channels 1740 (grooves) formed on at least one surface thereof, preferably the surface facing the die. (Generally, the die, rather than the substrate is the heat-generating element. In the case of disposing the interposer between two dies, cooling channels can be formed on both faces of the interposer.) When a cooling gas is directed at the assembly in the direction of the grooves, the gas flows through the grooves, removing heat generated by the die. This increases the reliability of the die, as well as of the connections made between the die and the interposer. In the embodiment of FIG. 17a, the interposer is substantially the same size (footprint) as the die.

FIG. 17b is a perspective view of an alternate embodiment of the assembly of FIG. 17a. In this case, a flip-chip assembly 1700b utilizes the same die 1720 and substrate 1730 as in FIG. 17a, but employs an embossed interposer 1710b which extends beyond the outline of the die 1720. In other words, the interposer has a larger footprint than the die. Consequently, the grooves 1740 on the face of the interposer will extend beyond the outline of the die 1720. In the presence of a directed flow of cooling gas, the additional grooved area of the interposer 1710b serve to provide additional surface area for capturing the flow of cooling gases, and provides for more effective heat removal by the cooling gas. After a fashion, the grooves 1740 act similarly to the fins of a heat sink, in that greater groove area equates with greater cooling capability. As discussed hereinbelow, the cooling grooves in the interposer can benefit the die directly (by removing heat from same or indirectly (by performing certain heat intensive operations of the die--effectively relocating these functions from the die to the interposer by incorporation of active and/or passive circuit elements into the interposer itself).

In the case of requiring cooling grooves on both sides of the interposer, it should be understood that the grooves on one side of the interposer can be disposed at a different angle than the grooves on the other side of the interposer.

According to another aspect of the invention, an interposer can be used to form conductive plastic connections between a die and a substrate. FIGS. 18a-e show the process at various stages of completion.

FIG. 18a is a perspective view of an interposer 1810 (preformed planar structure) prior to formation of any conductive structures. The interposer 1810 is formed of an electrically insulating material and (generally, as with all of the interposer embodiments disclosed herein) has substantially planar top and bottom surfaces 1810a and 1810b, respectively. A plurality of through-holes 1812 (only ten, of potentially many, per side illustrated) extend through the interposer 1810 from its top surface 1810a to its bottom surface 1810b (extension of the holes through the structure indicated by dashed lines). At this point, the through-holes 1812 are open (unfilled). The through-holes are arranged to align with contact pads on a semiconductor die and corresponding pads a mating substrate.

FIG. 18b is a cross-sectional view 1800a of the interposer 1810 after a first processing step. A conductive plastic material 1820 (e.g., conductive epoxy) is forced through the holes 1812 of the interposer 1810 under pressure (indicated by force lines 1822), by any suitable means, such that "globs" 1825a of the conductive material 1820 extend from the bottom surface 1810b of the interposer 1810. Then, a "doctor's knife" 1830 (a sharp knife-edge moving in the direction shown by the arrow in the Figure, sometimes referred to as a "doctor blade") is used to "scape" off that portion of the conductive material 1820 above the top surface 1810a of the interposer 1810. This results in the structure 1800b shown in FIG. 18c where the excess conductive plastic material has been removed, leaving flush top surfaces 1825b on the conductive material in the through-holes 1812. Evidently, the globs 1825a of excess plastic protrude beyond the bottom surface of the interposer, at this stage. By controlling the flow of conductive plastic material into the through holes, the extent of the globs 1825a beyond the bottom surface of the interposer may be controlled. It is also possible that globs of excess height are trimmed, by any suitable means, so that the globs extend uniformly a given distance beyond the bottom surface of the interposer.

FIG. 18d shows the arrangement 1800c resulting from a next processing step. The interposer 1810 (as shown in FIG. 18c) is positioned over a semiconductor die (or substrate) 1840 such that the conductive globs 1825a are aligned with conductive pads 1848 on the die. The interposer 1810 is then pressed downward onto the die such that the conductive globs 1825a contact the pads and spread out to form conductive liquid seals 1825 therewith. Ordinarily, the pads 1848 will be recessed slightly below the surface of a top passivation layer 1845 on the die. The back pressure on the conductive material 1820 in the through holes 1812 resulting from pressing the interposer down onto a die downward causes the "column" of conductive material 1820 in the through holes of the interposer to move upward (as viewed), thereby causing the conductive material 1820 to extend above the top surface 1810a of the interposer 1810, resulting in raised bumps 1825d. These raised bumps 1825d are effective for making contact to another die or substrate. Compare, e.g., the embodiment of FIG. 6b, which has conductive material extending out of both surfaces of the interposer. In this case (i.e., the embodiment of FIGS. 18a-d), the through holes of the interposer are filled with a plastic conductive material which is initially trimmed flush with the top surface of the interposer and allowed to flow out of the bottom surface of the interposer and, upon assembling to a die (or substrate), the pressure of assembly forces the material protruding from the bottom surface partially back into the through holes of the interposer, thereby resulting in the material protruding from the top surface of the interposer for further connection to another die or substrate. This, of course, requires that the plastic conductive material filling the through holes be, initially, somewhat fluid, later to be solidified by curing.

FIG. 18e show the arrangement 1800d after a final step, the substrate (1850) to which the die is to be connected, is positioned over the raised bumps 1825d and pressed downward onto them such that they adhere to corresponding conductive pads on the surface 1850a of the substrate and are spread outward somewhat to form conductive liquid seals 1825e therewith. The conductive material 1820 is then cured to optimize conductivity and adhesion characteristics.

The plastic conductive material described herein, for filling the through holes of an interposer, is any suitable conductive material, such as conductive epoxy or silicon material. It is relatively easy to screen print or otherwise apply these materials to interposer structures so that the conductive material fills the holes of the interposer.

As mentioned hereinabove, the contact pads of a semiconductor die are typically recessed below the surface of the die. The pad structure of the die may be recessed on the order of from one to several microns below a surrounding passivation layer on the die. In these cases, the interposer is preferably provided with conductive material "overfilling" the through holes, protruding beyond the through holes in a controlled manner. The embodiments of FIGS. 6b and 18a-e are exemplary of this feature. Alternatively, as exemplified by FIG. 16, columnar structures are formed on the surface of the die (or substrate) to ensure robust electrical contact with the conductive material of (i.e., in the through holes of) the interposer. Such columns can be formed, for example, of copper. In some cases, as also exemplified by FIG. 16, the die can be held in contact with the interposer by any suitable mechanical means (clips, pressure plates, etc.), so that the die can be replaced, if necessary.

Various embodiments of the interposers disclosed herein can be provided with active and/or passive electrical components (circuit elements), as exemplified by FIG. 11c. These electrical components can be connected to each other, and can be connected to the conductive material in the through holes of the interposer. These circuit elements can also be connected to other circuits not associated with the die, including: the substrate, other dies by means of conductive films or wires, and the substrate in remote locations by means of conductive films or wires. By virtue of the capability of incorporating these circuit elements into the interposer itself, the interposer can create an electrical switch network isolating the die from the substrate, so that one, some, or all of the connections of the substrate to the die will take place through the circuit elements disposed within the interposer. When the connection path of the substrate to the die can be completely determined by these circuit elements within the interposer, it becomes possible that the interposer can function as an active and reprogrammable element, and "re-wiring" of the connections of the die to the substrate becomes possible, with appropriate "programming" of the "active" interposer. In this manner, it becomes possible to change the "image" of the die as "seen" by the substrate. By methods of programming well known in the industry (e.g., fuse, anti-fuse, floating gate, battery-backed memory, etc.) impressed on the interposer through normal inputs and outputs or, alternatively, through inputs and outputs of the interposer dedicated to interposer programming, the connection path through the interposer, between the die and the substrate, is subject to one-time or repeated programming. Additionally, active and passive elements can be incorporated into the input and output paths of the interposer. In this manner, the interposer can substantially modify the characteristics of both incoming and outgoing signals. Such an interposer can also alter the power and ground connections between the substrate and the die. For example, if the die requires more in the way of capacitive buffering of the power and ground connections, either active or passive circuit elements can be incorporated into the interposer to effect such buffering. Such an active interposer can also be of use in controlling the heating of the die due to normal power dissipation. For example, it is possible to relocate the high power inputs and outputs of the die from the die to the interposer. In this manner, major power-consuming elements may be relocated from the die to the interposer which (i.e., the interposer) is easier (in some cases) to cool. For example, the interposers of FIG. 17a and 17b are well-suited to being cooled, as well as to aiding in the cooling of a die mounted to the interposer. One or ordinary skill in the art to which this invention most nearly pertains will appreciate the advantages and possibilities of incorporating active and passive electrical circuit elements into the interposer, based on the disclosure that has been made herein.

Among the various benefits discussed hereinabove, pertaining to the use of conductive plastic material (i.e., in the through holes of the interposer) for effecting connections from a die to another die or substrate, a great benefit accrues simply to obviating the need for using solder bumps to connect a die to a substrate. Solder bump processing is a complex and chemically "dirty" process. Note, for example, the efforts to control the application of flux, as described with respect to FIGS. 1-5. Further, the creation of solder bumps on a die or substrate by means of electroplating is particularly problematic because of the creating of plating "sludge" materials. These sludge materials are substantially toxic, and cannot be safely placed in toxic waste disposal sites because of their long term retention of toxicity.

The various embodiments of interposers described hereinabove are generally rather thin (although some of the embodiments are expressly thicker, and more mechanically substantial). Generally, control over the thickness of the interposer and the dimensions of the through holes (or cutouts) through the interposer are dictated by the particular application where it is intended to employ the interposer (e.g., such as by the parameters of the die to be bonded to the interposer). It should be understood that various of the embodiments of interposers set forth hereinabove can be manufactured as single, discrete units, or can be formed as a ribbon or tape of many units to be singulated.

Generally, the features of the various interposers discussed hereinabove can be "mixed and matched" to suit particular applications. For example, through holes in the ring like-interposer of FIG. 10a could be provided with an anti-galvanic foil as discussed with respect to FIG. 12. By way of further example, the probe fingers of FIG. 11a can be used in combination with the conductive plastic contact structures of FIG. 18e. Many other combinations of features, some of which have been expressly set forth hereinabove, are within the spirit and scope of this invention. 

What is claimed is:
 1. A preformed planar structure for flip-chip assembly, comprising:a planar base material, sized and shaped to fit between a chip and a substrate in a flip-chip assembly, and having two opposite faces, one face in contact with the chip and the opposite face in contact with the substrate; means for forming electrical connections between the chip and substrate through the planar base material; at least one first conductive element embedded within the planar base material; at least one second conductive element embedded within the planar base material; means for forming a further connection between the at least one first conductive element and at least one connection between the chip and the substrate; means for forming a further connection between the at least one second conductive element and at least one other connection between the chip and the substrate; means for electrically coupling the at least one first conductive element and the at least one second conductive element; means for forming selective electrical closure between the chip and the substrate by embedding at least one electronic switch within the planar base material; and a memory device embedded within the planar base material for retaining the state of electrical closure of the at least one electronic switch.
 2. A preformed planar structure according to claim 1, wherein the means for electrically coupling further comprises:at least one programming hole through the planar base material; a portion of the at least one first conductive element extending into the at least one programming hole: a portion of the at least one second conductive element extending into the at least one programming hole, but spaced apart from the portion of the at least one second conductive element; and means for effecting an electrical connection between the portion of the at least one first conductive element and the at least one second conductive element by filling the at least one programming hole with a conductive material.
 3. A preformed planar structure according to claim 1, wherein:the means for electrically coupling comprises electronic elements embedded within the planar base material to form at least one electronic switch between the at least one first conductive element and the at least one second conductive element.
 4. A preformed planar structure for flip-chip assembly, comprising:a planar base material sized and shaped to fit between a die and a substrate in a flip-chip assembly; means for forming electrical connections between the die and substrate through the planar base material; at least one first conductive element embedded within the planar base material; at least one second conductive element embedded within the planar base material; means for forming a further connection between the at least one first conductive element and at least one connection between the die and the substrate; means for forming a further connection between the at least one second conductive element and at least one other connection between the die and the substrate; means for altering a state of electrical connection between the at least one first conductive element and the at least one second conductive element; means for forming selective electrical closure between the die and the substrate by embedding at least one electronic switch within the planar base material; and a memory device embedded within the preformed planar structure for retaining the state of electrical closure of the at least one electronic switch.
 5. The preformed planar structure of claim 4, wherein:the at least one electronic switch is an FET.
 6. The preformed planar structure of claim 4, wherein:said memory device provides non-volatile storage of the state of electrical closure.
 7. The preformed planar structure of claim 6, further comprising:at least one programming hole for programming the at least one electronic switch.
 8. The preformed planar structure of claim 7, wherein:the at least one programming hole and the at least one electronic switch provide a programmable cross-point switching matrix within the planar base material.
 9. The preformed planar structure of claim 8, wherein:the cross-point switching matrix provides for a plurality of different interconnections. 